Truffle migrate importing OpenZeppelin Contracts results in Cannot read property "0" of null

Hello, I had the same problem as @pmk (Import OpenZeppelin Contracts on Windows: "Could not find import from any sources" error), it’s partially fixed but I have an error when I type the develop and migrate commands.

1 Like

Hi @Zee,

Welcome to the community :wave:

I’m sorry that you are having an issue. Are you able to share your smart contract so I can try to reproduce?

What versions of node/npm/truffle are you using? The easiest way to get these is to run
npx truffle version if truffle is installed locally, otherwise truffle version if truffle is installed globally.

This is what I am running currently:

$ npx truffle version
Truffle v5.1.29 (core: 5.1.29)
Solidity - ^0.6.0 (solc-js)
Node v10.19.0
Web3.js v1.2.1

I use Windows Subsystem for Linux (WSL2) and rarely have issues compared with native Windows. I would recommend having a look at using WSL: Solidity Smart Contract development on Windows

Hello, thank you for your answer :slight_smile:
Below is my smart contract:

// File: node_modules@openzeppelin\contracts\GSN\Context.sol

// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/*

  • @dev Provides information about the current execution context, including the

  • sender of the transaction and its data. While these are generally available

  • via msg.sender and msg.data, they should not be accessed in such a direct

  • manner, since when dealing with GSN meta-transactions the account sending and

  • paying for execution may not be the actual sender (as far as an application

  • is concerned).

  • This contract is only required for intermediate, library-like contracts.

*/

contract Context {

// Empty internal constructor, to prevent people from mistakenly deploying

// an instance of this contract, which should be used via inheritance.

constructor () internal { }

function _msgSender() internal view virtual returns (address payable) {

    return msg.sender;

}

function _msgData() internal view virtual returns (bytes memory) {

    this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691

    return msg.data;

}

}

// File: node_modules@openzeppelin\contracts\token\ERC20\IERC20.sol

// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/**

  • @dev Interface of the ERC20 standard as defined in the EIP.

*/

interface IERC20 {

/**

 * @dev Returns the amount of tokens in existence.

 */

function totalSupply() external view returns (uint256);

/**

 * @dev Returns the amount of tokens owned by `account`.

 */

function balanceOf(address account) external view returns (uint256);

/**

 * @dev Moves `amount` tokens from the caller's account to `recipient`.

 *

 * Returns a boolean value indicating whether the operation succeeded.

 *

 * Emits a {Transfer} event.

 */

function transfer(address recipient, uint256 amount) external returns (bool);

/**

 * @dev Returns the remaining number of tokens that `spender` will be

 * allowed to spend on behalf of `owner` through {transferFrom}. This is

 * zero by default.

 *

 * This value changes when {approve} or {transferFrom} are called.

 */

function allowance(address owner, address spender) external view returns (uint256);

/**

 * @dev Sets `amount` as the allowance of `spender` over the caller's tokens.

 *

 * Returns a boolean value indicating whether the operation succeeded.

 *

 * IMPORTANT: Beware that changing an allowance with this method brings the risk

 * that someone may use both the old and the new allowance by unfortunate

 * transaction ordering. One possible solution to mitigate this race

 * condition is to first reduce the spender's allowance to 0 and set the

 * desired value afterwards:

 * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729

 *

 * Emits an {Approval} event.

 */

function approve(address spender, uint256 amount) external returns (bool);

/**

 * @dev Moves `amount` tokens from `sender` to `recipient` using the

 * allowance mechanism. `amount` is then deducted from the caller's

 * allowance.

 *

 * Returns a boolean value indicating whether the operation succeeded.

 *

 * Emits a {Transfer} event.

 */

function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);

/**

 * @dev Emitted when `value` tokens are moved from one account (`from`) to

 * another (`to`).

 *

 * Note that `value` may be zero.

 */

event Transfer(address indexed from, address indexed to, uint256 value);

/**

 * @dev Emitted when the allowance of a `spender` for an `owner` is set by

 * a call to {approve}. `value` is the new allowance.

 */

event Approval(address indexed owner, address indexed spender, uint256 value);

}

// File: node_modules@openzeppelin\contracts\math\SafeMath.sol

// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/**

  • @dev Wrappers over Solidity’s arithmetic operations with added overflow

  • checks.

  • Arithmetic operations in Solidity wrap on overflow. This can easily result

  • in bugs, because programmers usually assume that an overflow raises an

  • error, which is the standard behavior in high level programming languages.

  • SafeMath restores this intuition by reverting the transaction when an

  • operation overflows.

  • Using this library instead of the unchecked operations eliminates an entire

  • class of bugs, so it’s recommended to use it always.

*/

library SafeMath {

/**

 * @dev Returns the addition of two unsigned integers, reverting on

 * overflow.

 *

 * Counterpart to Solidity's `+` operator.

 *

 * Requirements:

 * - Addition cannot overflow.

 */

function add(uint256 a, uint256 b) internal pure returns (uint256) {

    uint256 c = a + b;

    require(c >= a, "SafeMath: addition overflow");

    return c;

}

/**

 * @dev Returns the subtraction of two unsigned integers, reverting on

 * overflow (when the result is negative).

 *

 * Counterpart to Solidity's `-` operator.

 *

 * Requirements:

 * - Subtraction cannot overflow.

 */

function sub(uint256 a, uint256 b) internal pure returns (uint256) {

    return sub(a, b, "SafeMath: subtraction overflow");

}

/**

 * @dev Returns the subtraction of two unsigned integers, reverting with custom message on

 * overflow (when the result is negative).

 *

 * Counterpart to Solidity's `-` operator.

 *

 * Requirements:

 * - Subtraction cannot overflow.

 */

function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {

    require(b <= a, errorMessage);

    uint256 c = a - b;

    return c;

}

/**

 * @dev Returns the multiplication of two unsigned integers, reverting on

 * overflow.

 *

 * Counterpart to Solidity's `*` operator.

 *

 * Requirements:

 * - Multiplication cannot overflow.

 */

function mul(uint256 a, uint256 b) internal pure returns (uint256) {

    // Gas optimization: this is cheaper than requiring 'a' not being zero, but the

    // benefit is lost if 'b' is also tested.

    // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522

    if (a == 0) {

        return 0;

    }

    uint256 c = a * b;

    require(c / a == b, "SafeMath: multiplication overflow");

    return c;

}

/**

 * @dev Returns the integer division of two unsigned integers. Reverts on

 * division by zero. The result is rounded towards zero.

 *

 * Counterpart to Solidity's `/` operator. Note: this function uses a

 * `revert` opcode (which leaves remaining gas untouched) while Solidity

 * uses an invalid opcode to revert (consuming all remaining gas).

 *

 * Requirements:

 * - The divisor cannot be zero.

 */

function div(uint256 a, uint256 b) internal pure returns (uint256) {

    return div(a, b, "SafeMath: division by zero");

}

/**

 * @dev Returns the integer division of two unsigned integers. Reverts with custom message on

 * division by zero. The result is rounded towards zero.

 *

 * Counterpart to Solidity's `/` operator. Note: this function uses a

 * `revert` opcode (which leaves remaining gas untouched) while Solidity

 * uses an invalid opcode to revert (consuming all remaining gas).

 *

 * Requirements:

 * - The divisor cannot be zero.

 */

function div(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {

    // Solidity only automatically asserts when dividing by 0

    require(b > 0, errorMessage);

    uint256 c = a / b;

    // assert(a == b * c + a % b); // There is no case in which this doesn't hold

    return c;

}

/**

 * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),

 * Reverts when dividing by zero.

 *

 * Counterpart to Solidity's `%` operator. This function uses a `revert`

 * opcode (which leaves remaining gas untouched) while Solidity uses an

 * invalid opcode to revert (consuming all remaining gas).

 *

 * Requirements:

 * - The divisor cannot be zero.

 */

function mod(uint256 a, uint256 b) internal pure returns (uint256) {

    return mod(a, b, "SafeMath: modulo by zero");

}

/**

 * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),

 * Reverts with custom message when dividing by zero.

 *

 * Counterpart to Solidity's `%` operator. This function uses a `revert`

 * opcode (which leaves remaining gas untouched) while Solidity uses an

 * invalid opcode to revert (consuming all remaining gas).

 *

 * Requirements:

 * - The divisor cannot be zero.

 */

function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {

    require(b != 0, errorMessage);

    return a % b;

}

}

// File: node_modules@openzeppelin\contracts\utils\Address.sol

// SPDX-License-Identifier: MIT

pragma solidity ^0.6.2;

/**

  • @dev Collection of functions related to the address type

*/

library Address {

/**

 * @dev Returns true if `account` is a contract.

 *

 * [IMPORTANT]

 * ====

 * It is unsafe to assume that an address for which this function returns

 * false is an externally-owned account (EOA) and not a contract.

 *

 * Among others, `isContract` will return false for the following

 * types of addresses:

 *

 *  - an externally-owned account

 *  - a contract in construction

 *  - an address where a contract will be created

 *  - an address where a contract lived, but was destroyed

 * ====

 */

function isContract(address account) internal view returns (bool) {

    // According to EIP-1052, 0x0 is the value returned for not-yet created accounts

    // and 0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470 is returned

    // for accounts without code, i.e. `keccak256('')`

    bytes32 codehash;

    bytes32 accountHash = 0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470;

    // solhint-disable-next-line no-inline-assembly

    assembly { codehash := extcodehash(account) }

    return (codehash != accountHash && codehash != 0x0);

}

/**

 * @dev Replacement for Solidity's `transfer`: sends `amount` wei to

 * `recipient`, forwarding all available gas and reverting on errors.

 *

 * https://eips.ethereum.org/EIPS/eip-1884[EIP1884] increases the gas cost

 * of certain opcodes, possibly making contracts go over the 2300 gas limit

 * imposed by `transfer`, making them unable to receive funds via

 * `transfer`. {sendValue} removes this limitation.

 *

 * https://diligence.consensys.net/posts/2019/09/stop-using-soliditys-transfer-now/[Learn more].

 *

 * IMPORTANT: because control is transferred to `recipient`, care must be

 * taken to not create reentrancy vulnerabilities. Consider using

 * {ReentrancyGuard} or the

 * https://solidity.readthedocs.io/en/v0.5.11/security-considerations.html#use-the-checks-effects-interactions-pattern[checks-effects-interactions pattern].

 */

function sendValue(address payable recipient, uint256 amount) internal {

    require(address(this).balance >= amount, "Address: insufficient balance");

    // solhint-disable-next-line avoid-low-level-calls, avoid-call-value

    (bool success, ) = recipient.call{ value: amount }("");

    require(success, "Address: unable to send value, recipient may have reverted");

}

}

// File: node_modules@openzeppelin\contracts\token\ERC20\ERC20.sol

// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/**

  • @dev Implementation of the {IERC20} interface.

  • This implementation is agnostic to the way tokens are created. This means

  • that a supply mechanism has to be added in a derived contract using {_mint}.

  • For a generic mechanism see {ERC20MinterPauser}.

  • TIP: For a detailed writeup see our guide

  • https://forum.zeppelin.solutions/t/how-to-implement-erc20-supply-mechanisms/226[How

  • to implement supply mechanisms].

  • We have followed general OpenZeppelin guidelines: functions revert instead

  • of returning false on failure. This behavior is nonetheless conventional

  • and does not conflict with the expectations of ERC20 applications.

  • Additionally, an {Approval} event is emitted on calls to {transferFrom}.

  • This allows applications to reconstruct the allowance for all accounts just

  • by listening to said events. Other implementations of the EIP may not emit

  • these events, as it isn’t required by the specification.

  • Finally, the non-standard {decreaseAllowance} and {increaseAllowance}

  • functions have been added to mitigate the well-known issues around setting

  • allowances. See {IERC20-approve}.

*/

contract ERC20 is Context, IERC20 {

using SafeMath for uint256;

using Address for address;

mapping (address => uint256) private _balances;

mapping (address => mapping (address => uint256)) private _allowances;

uint256 private _totalSupply;

string private _name;

string private _symbol;

uint8 private _decimals;

/**

 * @dev Sets the values for {name} and {symbol}, initializes {decimals} with

 * a default value of 18.

 *

 * To select a different value for {decimals}, use {_setupDecimals}.

 *

 * All three of these values are immutable: they can only be set once during

 * construction.

 */

constructor (string memory name, string memory symbol) public {

    _name = name;

    _symbol = symbol;

    _decimals = 18;

}

/**

 * @dev Returns the name of the token.

 */

function name() public view returns (string memory) {

    return _name;

}

/**

 * @dev Returns the symbol of the token, usually a shorter version of the

 * name.

 */

function symbol() public view returns (string memory) {

    return _symbol;

}

/**

 * @dev Returns the number of decimals used to get its user representation.

 * For example, if `decimals` equals `2`, a balance of `505` tokens should

 * be displayed to a user as `5,05` (`505 / 10 ** 2`).

 *

 * Tokens usually opt for a value of 18, imitating the relationship between

 * Ether and Wei. This is the value {ERC20} uses, unless {_setupDecimals} is

 * called.

 *

 * NOTE: This information is only used for _display_ purposes: it in

 * no way affects any of the arithmetic of the contract, including

 * {IERC20-balanceOf} and {IERC20-transfer}.

 */

function decimals() public view returns (uint8) {

    return _decimals;

}

/**

 * @dev See {IERC20-totalSupply}.

 */

function totalSupply() public view override returns (uint256) {

    return _totalSupply;

}

/**

 * @dev See {IERC20-balanceOf}.

 */

function balanceOf(address account) public view override returns (uint256) {

    return _balances[account];

}

/**

 * @dev See {IERC20-transfer}.

 *

 * Requirements:

 *

 * - `recipient` cannot be the zero address.

 * - the caller must have a balance of at least `amount`.

 */

function transfer(address recipient, uint256 amount) public virtual override returns (bool) {

    _transfer(_msgSender(), recipient, amount);

    return true;

}

/**

 * @dev See {IERC20-allowance}.

 */

function allowance(address owner, address spender) public view virtual override returns (uint256) {

    return _allowances[owner][spender];

}

/**

 * @dev See {IERC20-approve}.

 *

 * Requirements:

 *

 * - `spender` cannot be the zero address.

 */

function approve(address spender, uint256 amount) public virtual override returns (bool) {

    _approve(_msgSender(), spender, amount);

    return true;

}

/**

 * @dev See {IERC20-transferFrom}.

 *

 * Emits an {Approval} event indicating the updated allowance. This is not

 * required by the EIP. See the note at the beginning of {ERC20};

 *

 * Requirements:

 * - `sender` and `recipient` cannot be the zero address.

 * - `sender` must have a balance of at least `amount`.

 * - the caller must have allowance for ``sender``'s tokens of at least

 * `amount`.

 */

function transferFrom(address sender, address recipient, uint256 amount) public virtual override returns (bool) {

    _transfer(sender, recipient, amount);

    _approve(sender, _msgSender(), _allowances[sender][_msgSender()].sub(amount, "ERC20: transfer amount exceeds allowance"));

    return true;

}

/**

 * @dev Atomically increases the allowance granted to `spender` by the caller.

 *

 * This is an alternative to {approve} that can be used as a mitigation for

 * problems described in {IERC20-approve}.

 *

 * Emits an {Approval} event indicating the updated allowance.

 *

 * Requirements:

 *

 * - `spender` cannot be the zero address.

 */

function increaseAllowance(address spender, uint256 addedValue) public virtual returns (bool) {

    _approve(_msgSender(), spender, _allowances[_msgSender()][spender].add(addedValue));

    return true;

}

/**

 * @dev Atomically decreases the allowance granted to `spender` by the caller.

 *

 * This is an alternative to {approve} that can be used as a mitigation for

 * problems described in {IERC20-approve}.

 *

 * Emits an {Approval} event indicating the updated allowance.

 *

 * Requirements:

 *

 * - `spender` cannot be the zero address.

 * - `spender` must have allowance for the caller of at least

 * `subtractedValue`.

 */

function decreaseAllowance(address spender, uint256 subtractedValue) public virtual returns (bool) {

    _approve(_msgSender(), spender, _allowances[_msgSender()][spender].sub(subtractedValue, "ERC20: decreased allowance below zero"));

    return true;

}

/**

 * @dev Moves tokens `amount` from `sender` to `recipient`.

 *

 * This is internal function is equivalent to {transfer}, and can be used to

 * e.g. implement automatic token fees, slashing mechanisms, etc.

 *

 * Emits a {Transfer} event.

 *

 * Requirements:

 *

 * - `sender` cannot be the zero address.

 * - `recipient` cannot be the zero address.

 * - `sender` must have a balance of at least `amount`.

 */

function _transfer(address sender, address recipient, uint256 amount) internal virtual {

    require(sender != address(0), "ERC20: transfer from the zero address");

    require(recipient != address(0), "ERC20: transfer to the zero address");

    _beforeTokenTransfer(sender, recipient, amount);

    _balances[sender] = _balances[sender].sub(amount, "ERC20: transfer amount exceeds balance");

    _balances[recipient] = _balances[recipient].add(amount);

    emit Transfer(sender, recipient, amount);

}

/** @dev Creates `amount` tokens and assigns them to `account`, increasing

 * the total supply.

 *

 * Emits a {Transfer} event with `from` set to the zero address.

 *

 * Requirements

 *

 * - `to` cannot be the zero address.

 */

function _mint(address account, uint256 amount) internal virtual {

    require(account != address(0), "ERC20: mint to the zero address");

    _beforeTokenTransfer(address(0), account, amount);

    _totalSupply = _totalSupply.add(amount);

    _balances[account] = _balances[account].add(amount);

    emit Transfer(address(0), account, amount);

}

/**

 * @dev Destroys `amount` tokens from `account`, reducing the

 * total supply.

 *

 * Emits a {Transfer} event with `to` set to the zero address.

 *

 * Requirements

 *

 * - `account` cannot be the zero address.

 * - `account` must have at least `amount` tokens.

 */

function _burn(address account, uint256 amount) internal virtual {

    require(account != address(0), "ERC20: burn from the zero address");

    _beforeTokenTransfer(account, address(0), amount);

    _balances[account] = _balances[account].sub(amount, "ERC20: burn amount exceeds balance");

    _totalSupply = _totalSupply.sub(amount);

    emit Transfer(account, address(0), amount);

}

/**

 * @dev Sets `amount` as the allowance of `spender` over the `owner`s tokens.

 *

 * This is internal function is equivalent to `approve`, and can be used to

 * e.g. set automatic allowances for certain subsystems, etc.

 *

 * Emits an {Approval} event.

 *

 * Requirements:

 *

 * - `owner` cannot be the zero address.

 * - `spender` cannot be the zero address.

 */

function _approve(address owner, address spender, uint256 amount) internal virtual {

    require(owner != address(0), "ERC20: approve from the zero address");

    require(spender != address(0), "ERC20: approve to the zero address");

    _allowances[owner][spender] = amount;

    emit Approval(owner, spender, amount);

}

/**

 * @dev Sets {decimals} to a value other than the default one of 18.

 *

 * WARNING: This function should only be called from the constructor. Most

 * applications that interact with token contracts will not expect

 * {decimals} to ever change, and may work incorrectly if it does.

 */

function _setupDecimals(uint8 decimals_) internal {

    _decimals = decimals_;

}

/**

 * @dev Hook that is called before any transfer of tokens. This includes

 * minting and burning.

 *

 * Calling conditions:

 *

 * - when `from` and `to` are both non-zero, `amount` of ``from``'s tokens

 * will be to transferred to `to`.

 * - when `from` is zero, `amount` tokens will be minted for `to`.

 * - when `to` is zero, `amount` of ``from``'s tokens will be burned.

 * - `from` and `to` are never both zero.

 *

 * To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].

 */

function _beforeTokenTransfer(address from, address to, uint256 amount) internal virtual { }

}

My versions of node/npm/truffle:

Truffle v5.1.29 (core: 5.1.29)
Solidity v0.5.16 (solc-js)
Node v12.18.0
Web3.js v1.2.1

I already have WSL2 on my computer and I make the same mistake when I use it.

Thank you for your help.

1 Like

Hi @zee,

Are you still having the issue?

The code you posted is a flattened ERC20 contract but appeared to be truncated.
It is much easier to read a contract with imports than a flattened version.

If you are just creating a simple ERC20, you may want to look at using a Preset contract: Create an ERC20 using Truffle, without writing Solidity