What is the right way to check for a null string in Objective-C?

I was using this in my iPhone app

if (title == nil) {
    // do something
}

but it throws some exception, and the console shows that the title is “(null)”.

So I’m using this now:

if (title == nil || [title isKindOfClass:[NSNull class]]) {
    //do something
}

What is the difference, and what is the best way to determine whether a string is null?