diff --git a/reentrancy-warmup/vulnerable.sol b/reentrancy-warmup/vulnerable.sol
index 0218f2c..630ba5e 100644
--- a/reentrancy-warmup/vulnerable.sol
+++ b/reentrancy-warmup/vulnerable.sol
@@ -38,7 +38,7 @@ contract TestToken {
 
     function withdraw(uint256 _value) payable returns (bool success) {
         if (balances[msg.sender] < _value) return false;
-        msg.sender.call.value(_value)();
+        msg.sender.transfer(_value);
         balances[msg.sender] -= _value;
         total -= _value;
         return true;