Hello, I was doing an auction house auctionning an NFT (ERC721) every day. If there is a bidder, then the NFT is transfered at the end of the auction by calling settleCurrentAndCreateNewAuction(), and if there are no bidders, the auction house (the owner of the current NFT) should burn the NFT.
The thing is that the auction house can't burn the NFT for whatever reason I get this error when running settleCurrentAndCreateNewAuction() with no bidders : Error: Transaction reverted: function returned an unexpected amount of data.
After using lots of console.log via hardhat, I noticed that this function was the problem :
function _burn(uint256 tokenId) internal virtual {
address owner = ERC721.ownerOf(tokenId);
_beforeTokenTransfer(owner, address(0), tokenId);
// Clear approvals
_approve(address(0), tokenId);
_balances[owner] -= 1;
delete _owners[tokenId];
emit Transfer(owner, address(0), tokenId);
}
I don't really understand where this comes from as the function doesn't return anything, here is my NFT contract :
//SPDX-License-Identifier: MIT
pragma solidity ^0.8.6;
import { ERC721 } from "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import { Ownable } from "@openzeppelin/contracts/access/Ownable.sol";
import "base64-sol/base64.sol";
/// @title Economics Design Property Right NFT.
/// @notice Contract used to verify the ownership of the Economics Design book property rights.
contract EconNFT is ERC721, Ownable {
// expiration timestamp of the NFT, date after the owner won't be able to buy books anymore.
struct Seed {
uint256 expirationTimestamp;
}
// Minter of the EconNFT.
address public minter;
// Id currently auctionned.
uint256 private _currentEconNFTId;
bool public isMinterLocked;
// Contain information about the NFT that are set when its first minted.
mapping(uint256 => Seed) public seeds;
// Expiration timestamp currently associated with each EconNFT.
uint256 public currentExpirationTimestamp;
// Constant used to translate a timestamp into a date for tokenURI().
uint constant SECONDS_PER_DAY = 24 * 60 * 60;
// Constant used to translate a timestamp into a date for tokenURI() as timestamps start from 01/01/1970.
int constant OFFSET19700101 = 2440588;
// Variables used to make the JPEG image.
uint256 public maxNumberOfPath;
uint256 public maxNumberOfPathCommands;
uint256 public size;
string[] public pathCommands;
string[] public colors;
/// @param _totalSupply maximum number of NFT that is going to be minted.
/// @param _expirationTimestamp the date where the EconNFT will expire (timestamp in seconds).
/// @dev Variables used to make the JPEG are set in order to construct an SVG later on.
constructor(uint256 _totalSupply, uint256 _expirationTimestamp) ERC721("Econteric IP", "ECIP") Ownable() public {
isMinterLocked = false;
_currentEconNFTId = 0;
currentExpirationTimestamp = _expirationTimestamp;
maxNumberOfPath = 10;
maxNumberOfPathCommands = 5;
size = 500;
pathCommands = ["M", "L"];
colors = ["red", "blue", "green", "yellow", "black", "white"];
}
/// @notice Restrict a function only when minter is not locked (i.e. isMinterLocked == false).
modifier whenMinterNotLocked() {
require(!isMinterLocked, "Minter is locked");
_;
}
/// @notice Restrict a function to be called only by the minter address.
modifier onlyMinter() {
require(msg.sender == minter, "Sender is not the minter");
_;
}
/// @notice Mint a new NFT.
/// @return Return the id of the minted NFT.
/// @dev Create a new NFT and send it to the minter.
function mint() public onlyMinter returns (uint256) {
return _mintTo(minter, _currentEconNFTId++);
}
/// @notice Call the ERC721 `_burn()` function which burn an NFT and sends it to the address(0).
/// @param econNFTId the id of the NFT you want to burn.
function burn(uint256 econNFTId) public onlyMinter {
_burn(econNFTId);
}
/// @notice Mint an property right NFT.
/// @param to the address to send to the minted NFT.
/// @param econNFTId the id of the NFT to mint.
function _mintTo(address to, uint256 econNFTId) internal returns (uint256) {
seeds[econNFTId] = Seed({
expirationTimestamp: currentExpirationTimestamp
});
_mint(to, econNFTId);
return econNFTId;
}
/// @notice Contain all the informations needed for read this NFT informations.
/// @param tokenId the id of the token you would like to read information from.
/// @return data a 64 based encoded string containing a json object with all the informations.
function tokenURI(uint256 tokenId) public view override returns (string memory) {
uint256 year;
uint256 month;
uint256 day;
(year, month, day) = timestampToDate(seeds[tokenId].expirationTimestamp);
uint256 randomNumber = block.timestamp / block.number;
string memory svg = generateSVG(randomNumber);
string memory imageURI = svgToImageURI(svg);
string memory json = Base64.encode(bytes(abi.encodePacked(
'{ "name": "Econteric Book", ',
'"description": "Economics and Math of Token Engineering and DeFi", ',
'", "expirationDate": "',
uint2str(day),
'/',
uint2str(month),
'/',
uint2str(year),
'", "image": "',
imageURI,
'" }'
)));
string memory data = string(abi.encodePacked('data:application/json;base64,', json));
return data;
}
/// @notice Transform a uint type into a string type. Used to encode tokenURI().
/// @param _i the uint to encode into string.
/// @return _uintAsString the same number but in a string format.
/// @dev Function taken from https://github.com/provable-things/ethereum-api/blob/master/provableAPI_0.6.sol
function uint2str(uint _i) internal pure returns (string memory _uintAsString) {
if (_i == 0) {
return "0";
}
uint j = _i;
uint len;
while (j != 0) {
len++;
j /= 10;
}
bytes memory bstr = new bytes(len);
uint k = len;
while (_i != 0) {
k = k-1;
uint8 temp = (48 + uint8(_i - _i / 10 * 10));
bytes1 b1 = bytes1(temp);
bstr[k] = b1;
_i /= 10;
}
return string(bstr);
}
/// @notice Takes a timestamp in seconds and convert it into a human readable date.
/// @param timestamp the timestamp you want to convert.
/// @return year the year from your timestamp.
/// @return month the month from your timestamp (between 1 and 12).
/// @return day the day from your timestamp (between 1 and 31).
function timestampToDate(uint timestamp) internal pure returns (uint year, uint month, uint day) {
(year, month, day) = _daysToDate(timestamp / SECONDS_PER_DAY);
}
/// @notice Calculate year/month/day from the number of days _days since 1970/01/01.
/// @param _days the number of day between the timestamp you can to convert into a date and 01/01/1970.
/// @return year the year from your timestamp.
/// @return month the month from your timestamp (between 1 and 12).
/// @return day the day from your timestamp (between 1 and 31).
/// @dev Function taken from https://etherscan.io/address/0x78f96b2d5f717fa9ad416957b79d825cc4cce69d#code.
function _daysToDate(uint _days) internal pure returns (uint year, uint month, uint day) {
int __days = int(_days);
int L = __days + 68569 + OFFSET19700101;
int N = 4 * L / 146097;
L = L - (146097 * N + 3) / 4;
int _year = 4000 * (L + 1) / 1461001;
L = L - 1461 * _year / 4 + 31;
int _month = 80 * L / 2447;
int _day = L - 2447 * _month / 80;
L = _month / 11;
_month = _month + 2 - 12 * L;
_year = 100 * (N - 49) + _year + L;
year = uint(_year);
month = uint(_month);
day = uint(_day);
}
/// @notice Generate a full SVG component.
/// @param _randomNumber is a random number used to construct the SVG (the more random the better).
/// @return finalSvg the full SVG component readable by a browser.
/// @dev The random number is used to print random lines and random colors on the final SVG.
function generateSVG(uint256 _randomNumber) public view returns(string memory finalSvg) {
uint256 numberOfPath = (_randomNumber % maxNumberOfPath) + 1;
finalSvg = string(abi.encodePacked('<svg xmlns="http://www.w3.org/2000/svg" height="', uint2str(size), '" width="', uint2str(size), '">'));
for(uint i = 0; i < numberOfPath; i++) {
uint256 newRNG = uint256(keccak256(abi.encode(_randomNumber, i)));
string memory pathSvg = generatePath(newRNG);
finalSvg = string(abi.encodePacked(finalSvg, pathSvg));
}
finalSvg = string(abi.encodePacked(finalSvg, "</svg>"));
}
/// @notice Generate different paths that creates the SVG lines.
/// @param _randomNumber is a random number from the `generatePath()` function.
/// @return pathSvg a string containing all SVG instructions.
/// @dev Function called by the `generateSVG()` function.
function generatePath(uint256 _randomNumber) public view returns(string memory pathSvg) {
uint256 numberOfPathCommands = (_randomNumber % maxNumberOfPathCommands) + 1;
pathSvg = '<path d="';
for(uint i = 0; i < numberOfPathCommands; i++) {
uint256 newRNG = uint256(keccak256(abi.encode(_randomNumber, size + i)));
string memory pathCommand = generatePathCommand(newRNG);
pathSvg = string(abi.encodePacked(pathSvg, pathCommand));
}
string memory color = colors[_randomNumber % colors.length];
pathSvg = string(abi.encodePacked(pathSvg, '" fill="transparent" stroke="', color, '"/>'));
}
/// @notice Generate the paths to create the SVG.
/// @param _randomNumber is a random number from the `generatePath()` function.
/// @return pathCommand a string containing the paths to create the SVG.
/// @dev Called by the `generatePath()` function.
function generatePathCommand(uint256 _randomNumber) public view returns(string memory pathCommand) {
pathCommand = pathCommands[_randomNumber % pathCommands.length];
uint256 parameterOne = uint256(keccak256(abi.encode(_randomNumber, size * 2))) % size;
uint256 parameterTwo = uint256(keccak256(abi.encode(_randomNumber, size * 2 + 1))) % size;
pathCommand = string(abi.encodePacked(pathCommand, " ", uint2str(parameterOne), " ", uint2str(parameterTwo), " "));
}
/// @notice Takes a SVG and encode it while still keeping it readable by a browser.
/// @param _svg is the SVG component.
/// @return imageURI a string readable by the browser which will display the SVG.
function svgToImageURI(string memory _svg) public pure returns(string memory) {
string memory baseURL = "data:image/svg+xml;base64,";
string memory svgBase64Encoded = Base64.encode(bytes(string(abi.encodePacked(_svg))));
string memory imageURI = string(abi.encodePacked(baseURL, svgBase64Encoded));
return imageURI;
}
/// @notice Change the minter address.
/// @param _minter The new minter address.
function setMinter(address _minter) external onlyOwner whenMinterNotLocked {
minter = _minter;
}
/// @notice Lock the minter from calling certain functions.
function lockMinter() external onlyOwner whenMinterNotLocked {
isMinterLocked = true;
}
/// @notice Change the expiration timestamp of the future NFTs about to get minted.
/// @param _newExpirationTimestamp the new timestamp.
function setCurrentExpirationTimestamp(uint256 _newExpirationTimestamp) external onlyOwner {
currentExpirationTimestamp = _newExpirationTimestamp;
}
}
Thank you in advance for your help 
I hope someone already found and resolve a similar bug
