For loop issue

Support Contracts
1

I have a contract with the following functions:

contract A
{
   struct Data
   {
      uint x;
      uint y;
   }

   function buyItem(Data memory dat) public payable
   {
      // code to buy 1 item
   }

   function buyItems(Data[] memory data) public payable
   {
      uint nr_els = data.length;

      for( uint i = 0; i < nr_els; i++)
      {
         buyPixel(data[i]);
      }
   }

}

In the javascript test file I call this function as follows:

var data = [];
data[0] = dat1;
data[1] = dat2;

await instance.buyItems(data, {for: user1, value:10000});

When I run this test, I get the following error:

Error: Returned error: VM Exception while processing transaction: revert

I did some more testing. For example, if I initialize the for loop as for(int i = 1 ... ), so start from 1 instead of 0, then I don't get this error. Also if I just keep the original for loop ( starting at i = 0 ) and in the test file I also make the array var dat[] just 1 element long ( instead of 2 ), then I also don't get an error.

I have no idea why I am getting this error message. The error message doesn't give me any usefull information. Anyone else knows what might be happening here ? Or can someone point me out how I can get more usefull error information than this with more detailed data about the error it is complaining about ?