“Collection objects, such as NSArray, NSSet,and NSDictionary, can’t contain nil as a value. Instead, nil values are represented by a special object, NSNull. NSNull provides a single instance that represents the nil value for object properties. The default implementations of dictionaryWithValuesForKeys: and setValuesForKeysWithDictionary: translates between NSNull and nil automatically, so your objects don’t have to explicitly test for NSNull values.”

Source: “Key-Value Programming Guide”, page 13