Why base64EncodeData doesn't work?

NSString *result = [encData base64EncodeData:encData];  

why base64EncodeData does't 开发者_如何转开发work?? it had the message like:

-[NSConcreteData base64EncodeData:]: unrecognized selector sent to instance 0x4e1f020

There is no base64EncodeData method in the official SDK. If you are using third-party libraries/code that define categories on NSData/NSString for base64 encoding/decoding, be sure to add them to your project.