human
Solve the following python programming problem given the following code:
def on_recv(self, callback) : 
    "
        Register a callback for received messages (that we didn't initiate)

        :param func callback: A function which should be called when data is received
        " 
    if (self is None) : 
        return self.stream.on_recv (None) 
@ tornado.gen.coroutine 
    def wrap_callback(messages) : 
        payload = (yield self._decode_messages (messages)) 
        if (payload is not None) : 
            callback (payload) 
return self.stream.on_recv (wrap_callback) 

    
    

    Fix the buggy line: if (self is None) :
assistant
#Here is Python code to fix the buggy line: if (self is None) :
.

if (callback is None) :