静态objective-c类是否必须子类NSObject?

在我的objective-c项目中,我有一个奇怪的,比方说, 特征

#import <Foundation/Foundation.h>

@interface Convert /* : NSObject */ // <--- is that necessary? 

+(int) toInt:(id) obj;

@end

@implementation Convert

+(int) toInt:(id) obj
{
     return [obj intValue];
}

@end

发生的情况是,当我单步执行代码时,它工作正常,但控制台中出现了一个神秘的错误(尽管代码完全正常,但工作正常):

2010-11-03 09:35:49.422 Tests[14066:5f03] *** NSInvocation: warning: object 0x9e424 of class 'Convert' does not implement methodSignatureForSelector: -- trouble ahead
2010-11-03 09:35:49.422 Tests[14066:5f03] *** NSInvocation: warning: object 0x9e424 of class 'Convert' does not implement doesNotRecognizeSelector: -- abort