ios [__NSCFString MD5String]：无法识别的选择器发送到实例

从NSS计算md5
tring,一个类别已定义为：

NSString的-md5.h

@interface NSString (MD5)
- (NSString *)MD5String;
@end

NSString的-md5.m

#import <CommonCrypto/CommonDigest.h>
#import "NSString-md5.h"
@implementation NSString(MD5)
- (NSString *)MD5String
    {is
    const char *cstr = [self UTF8String];
    unsigned char result[16];    
    CC_MD5( cstr,strlen(cstr),result );
    return [NSString stringWithFormat:
        @"%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X",result[0],result[1],result[2],result[3],result[4],result[5],result[6],result[7],result[8],result[9],result[10],result[11],result[12],result[13],result[14],result[15]
        ];  
    }
@end

调用代码(提取)

#import "NSString-md5.h"
...
NSLog(@"1 %@ ",message.text);
[message.text MD5String];

中止并显示错误消息：

2013-03-03 19:06:14.104 coffreFort[1230:11303] 1 aze 
2013-03-03 19:06:22.777 coffreFort[1230:11303] -[__NSCFString MD5String]: unrecognized selector sent to instance 0x71e62b0
2013-03-03 19:06:22.777 coffreFort[1230:11303] *** Terminating app due to uncaught exception 'NSInvalidArgumentException',reason: '-[__NSCFString MD5String]: unrecognized selector sent to instance 0x71e62b0'

message.text是由IB定义的IBOutlet

@property (weak,nonatomic) IBOutlet UITextField *message;