Adding Categories to NSMutableData

FROM : Robert Kukuchka
DATE : Thu May 29 19:59:22 2008

Hello,
   I'm trying to add a simple category to NSMutableData, but when I try 
to call my method I'm getting "*** -[NSConcreteData 
remainderDataWithParseByte:]: unrecognized selector sent to instance 
0x816400". I've added categories to NSString and NSArray before, which 
have similar abstract interfaces to concrete classes. Is there some 
trick I'm missing?

Cheers
Rob