    function delegate(address delegatee) public {
        if (delegatee == address(0)) delegatee = msg.sender;
        return _delegate(msg.sender, delegatee);
    }
    function delegateBySig(
        address delegatee,
        uint256 nonce,
        uint256 expiry,
        uint8 v,
        bytes32 r,
        bytes32 s
    ) public {
        bytes32 domainSeparator = keccak256(
            abi.encode(DOMAIN_TYPEHASH, keccak256(bytes(name())), getChainId(), address(this))
        );
        bytes32 structHash = keccak256(abi.encode(DELEGATION_TYPEHASH, delegatee, nonce, expiry));
        bytes32 digest = keccak256(abi.encodePacked('\x19\x01', domainSeparator, structHash));
        address signatory = ecrecover(digest, v, r, s);
        require(signatory != address(0), 'ERC721Checkpointable::delegateBySig: invalid signature');
        require(nonce == nonces[signatory]++, 'ERC721Checkpointable::delegateBySig: invalid nonce');
        require(block.timestamp <= expiry, 'ERC721Checkpointable::delegateBySig: signature expired');
        return _delegate(signatory, delegatee);
    }
    function _delegate(address delegator, address delegatee) internal {
        /// @notice differs from `_delegate()` in `Comp.sol` to use `delegates` override method to simulate auto-delegation
        address currentDelegate = delegates(delegator);

        _delegates[delegator] = delegatee;
...
    function delegates(address delegator) public view returns (address) {
        address current = _delegates[delegator];
        return current == address(0) ? delegator : current;
    }
    function _moveDelegates(
        address srcRep,
        address dstRep,
        uint96 amount
    ) internal {
        if (srcRep != dstRep && amount > 0) {
            if (srcRep != address(0)) {
                uint32 srcRepNum = numCheckpoints[srcRep];
                uint96 srcRepOld = srcRepNum > 0 ? checkpoints[srcRep][srcRepNum - 1].votes : 0;
                uint96 srcRepNew = sub96(srcRepOld, amount, 'ERC721Checkpointable::_moveDelegates: amount underflows'); // auditor : overflow here
                _writeCheckpoint(srcRep, srcRepNum, srcRepOld, srcRepNew);
            }
    function burn(uint256 nounId) public override onlyMinter {
        _burn(nounId);
        emit NounBurned(nounId);
    }
...
    function _burn(uint256 tokenId) internal virtual {
        address owner = ERC721.ownerOf(tokenId);

        _beforeTokenTransfer(owner, address(0), tokenId);
...
    function _beforeTokenTransfer(
        address from,
        address to,
        uint256 tokenId
    ) internal override {
        super._beforeTokenTransfer(from, to, tokenId);

        /// @notice Differs from `_transferTokens()` to use `delegates` override method to simulate auto-delegation
        _moveDelegates(delegates(from), delegates(to), 1);
    }
    function delegateBySig(
        address delegatee,
        uint256 nonce,
        uint256 expiry,
        uint8 v,
        bytes32 r,
        bytes32 s
    ) public {
+ require(delegatee != address(0));
