Adding nil object to NSDictionary

NSDictionary and NSArray do not allow adding nil object to them.
But [NSNull null] can solve this problem in case you really want to have a "Null Object"

        if(anObject!=nil)
        {
     [self setObject:anObject forKey:aKey];
     return YES;
 }
 else
        {
     [self setObject:[NSNull null] forKey:aKey];
     return NO;
 }

reference: https://gist.github.com/307568