I have been following a course, and, I followed their code, somehow, when I compile my code, it gave me an error, and the course I followed didn't get an error, somebody help me please.

pragma solidity ^0.8.7;

import './ERC165.sol';
import './IERC721.sol';

contract ERC721 is ERC165, IERC721 {

        function ownerOf(uint256 _tokenId) public override view returns (address) {
            address owner = _tokenOwner[_tokenId];
            require(owner != address(0), 'owner query for non-existent token');
            return owner;

    // mapping in solidity creates a hash table of key pair values

    // Mapping from token id to the owner
    mapping(uint => address) private _tokenOwner;

    // Mapping from owner to number of owned tokens
    mapping(address => uint256) private _OwnedTokensCount;

    mapping(uint256 => address) private _tokenApprovals;

    constructor() {

    function balanceOf(address _owner) public override view returns (uint256) {
        require(_owner != address(0), 'owner query for non-existant token');
        return _OwnedTokensCount[_owner];
    function _exists(uint256 tokenId) internal view returns(bool) {
       // setting the address of nft owner to check the mapping
       // of the address from tokenOwner at the tokenId
       address owner = _tokenOwner[tokenId]; 
       // return truthiness tha address is not zero
       return owner != address(0);

    function _mint(address to, uint256 tokenId) internal virtual {
        require(to != address(0), 'ERC721: minting to the zero address');
        require(!_exists(tokenId), 'ERC721: token already minted');
        _tokenOwner[tokenId] = to;
        _OwnedTokensCount[to] += 1;

        emit Transfer(address(0), to, tokenId);

    function _transferFrom(address _from, address _to, uint256 _tokenId) external payable
        require(_to != address(0), 'Error - ERC721 transfer to the zero address');
        require(ownerOf(_tokenId) == _from, 'Trying to transfer a token the address does not own!');

        _OwnedTokensCount[_from] -= 1;
        _OwnedTokensCount[_from] += 1;
        _tokenOwner[_tokenId] = _to;

        emit Transfer(_from, _to, _tokenId);

    function transferFrom(address _from, address _to, uint256 _tokenId) override public {
        require(msg.sender, _tokenId);
        transferFrom(_from, _to, _tokenId);

    function approve(address _to, uint256 tokenId) public {
        address owner = ownerOf(tokenId);
        require(_to != owner, 'Error - approval to current owner');
        require(msg.sender == owner, 'Current caller is not the owner of the token');
        _tokenApprovals[tokenId] = _to;
        emit Approval(owner, _to, tokenId);

the error was on the function called transferfrom it gave me an error saying [1]: https://i.stack.imgur.com/m3Eio.png

please help me, I've been stuck here for an hour, I don't know what to do.

  • What do you mean by this: require(msg.sender, _tokenId); ? Jul 15, 2022 at 6:21
  • what do you mean by that?
    – Korumaq1xa
    Jul 15, 2022 at 6:34

1 Answer 1


This code is wrong:

function transferFrom(address _from, address _to, uint256 _tokenId) override public {
        require(msg.sender, _tokenId);
        transferFrom(_from, _to, _tokenId);

should be:

function transferFrom(address _from, address _to, uint256 _tokenId) override public {
    require(msg.sender == _tokenOwner[_tokenId]," Error, not the owner"); 
    transferFrom(_from, _to, _tokenId);

There should be judgment in require, you can change it to other


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