Damn Vulnerable DeFi - Unstoppable

unstoppable4.sol

IERC20 public damnValuableToken;
uint256 public poolBalance;

constructor(address tokenAddress) public {
    require(tokenAddress != address(0), "Token address cannot be zero");
    damnValuableToken = IERC20(tokenAddress);
}

function depositTokens(uint256 amount) external nonReentrant {
    require(amount > 0, "Must deposit at least one token");
    // Transfer token from sender. Sender must have first approved them.
    damnValuableToken.transferFrom(msg.sender, address(this), amount);
    poolBalance = poolBalance.add(amount);
}