Ok looks like I tracked down the problem. I realized the compiler was sending out warnings:
incompatible pointer types initializing 'NSMutableData *' with an expression of type 'NSData *'
It turns out I had some code
NSMutableData *message = [data subdataWithRange:NSMakeRange(5, len - 5)];
Which i needed to convert into:
NSMutableData *message = [NSMutableData dataWithData:[data subdataWithRange:NSMakeRange(5, len - 5)]];
And then things all work out. Moral of the story (read your warnings!!!!)