diff --git a/contracts/token/StandardToken.sol b/contracts/token/StandardToken.sol
index ee499b964..1eb4e114a 100644
--- a/contracts/token/StandardToken.sol
+++ b/contracts/token/StandardToken.sol
@@ -29,6 +29,13 @@ contract StandardToken is BasicToken, ERC20 {
   }
 
   function approve(address _spender, uint _value) {
+
+    // To change the approve amount you first have to reduce the addresses`
+    //  allowance to zero by calling `approve(_spender,0)` if it is not
+    //  already 0 to mitigate the race condition described here:
+    //  https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
+    if ((_amount!=0) && (allowed[msg.sender][_spender] !=0)) throw;
+
     allowed[msg.sender][_spender] = _value;
     Approval(msg.sender, _spender, _value);
   }