According to http://developer.apple.com/documentation/Cocoa/Conceptual/CocoaFundamentals/CocoaObjects/chapter_3_section_7.html#//apple_ref/doc/uid/TP40002974-CH4-DontLinkElementID_93 "In all isEqualToType: methods of the Cocoa frameworks nil is not a valid argument and implementations of these methods may raise an exception upon receiving a nil." However, this warning is rarely if ever presented in the documentation for these methods in the class reference. For example, the warning is missing at http://developer.apple.com/documentation/Cocoa/Reference/Foundation/Classes/NSValue_Class/Reference/Reference.html#//apple_ref/doc/uid/20000177-isEqualToValue_