Checking if a key in standardUserDefaults has been set or is NSNull

What is the correct way to check the contents of standardUserDefaults ?

If I print out what is stored in a key i get:

NSLog(@"logged_in_status : %@", [standardUserDefaults stringForKey:@"logged_in_status"]);
Output: logged_in_status : (null)

So it seems to be NSNull.

So I will add a check for that as follows:

 if ([[standardUserDefaults objectForKey:@"logged_in_status"] isKindOfClass:[NSNull class]])
        {
            NSLog(@"YES");
        }
        else
        {
            NSLog(@"NO");
        }

Output: NO

Why does this not resolve to the YES? Isn't it an NSNull?

I've been looking at this for a long time and can't see where the error is. Could anybody kindly explain where I've gone wrong on this?

Thanks, Code