Three20 crash when using PhotoSource

Question

Hi all, In my app I send a request to get images from flickr. I have went through my app step-by-step with a breakpoint and found the app crashes at this line in TTURLRequest.m:

} else { 

    return [self.urlPath md5Hash]; 
  } 

I get this error in the debugger:

* Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSCFString md5Hash]: unrecognized selector sent to instance 0x74409e0'

Does anybody know what is causing this? Thanks