    function _mint(address account, uint256 amount) internal virtual {
        _notZero(account);
+       _notZero(amount);
        assert(totalStakes + amount < type(uint224).max);

        stakes[era][account] += amount;
        totalStakes += amount;

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