title: NSString containsString crashes
body: <p>I'm trying to filter an array according to one of it's string fields.</p>
  
  <p>Both nameLower and filterLower has NSString value inside, and yet i keep getting:</p>
  
  <pre><code>__NSCFString containsString:]: unrecognized selector sent to instance 0x7f876b79e160
  
  -(void) filterFriendsArray:(NSString*)filter {
      [_filteredFriendsArray removeAllObjects];
      for (FacebookUser* user in _friendsArray)
      {
          NSString* nameLower = [user.user.name lowercaseString];
          NSString* filterLower = [filter lowercaseString];
          if ([nameLower containsString:filterLower])
              [_filteredFriendsArray addObject:user];
      }
      _displayedFriendsArray = _filteredFriendsArray;
  }
  </code></pre>