.net - C# - Code Contracts - Detected Expression Statement Evaluated For Potential Side-effect?

I have a class with a static list of keys, and I have a static method called Remove() that removes a key from that list.Now what I understand from the contracts post conditions is that as the programmer of the class, I guarantee that the current public method does something specific, in this case : removing the element of the list.

Here's what I've written:

private static List<Keys> m_usedKeys; // private list of keys
public static void Remove(Keys _key)
{

[code]....

What I'm trying to "Ensure" here, is that the Remove() method removes the key for real from the list (Yes this might be a trivial example, but I get the error there.)when I write the Contract.Ensures() line, VS2010 gives me the following error :

Error 3 Detected expression statement evaluated for potential side-effect in contracts of method 'LibJungleTimer.KeyBind.Remove(System.Windows.Forms.Keys)'. (Did you mean to put the expression into a Requires, Ensures, or Invariant call?) C:UsersJoelDocumentsProgrammationJTJungleTimerLibJungleTimerKeyBind.cs 51

I think this error says that the expression m_usedKeys.remove(_key); has a potential side-effect. In fact, it has a side-effect, it removes a key from the list!If I try to write something like this instead :

Contract.Assert(!m_usedKeys.Any(x => x == _key));

well it compiles fine.