human
Solve the following python programming problem given the following code:
def lastupdate(self, filename, defer = None) : 
    if (defer is None) : 
        defer = self._defer 
doinfo = (lambda : super (RRDTwistedAPI, self).lastupdate (filename)) 
    def merge(pending) : 
        def returnmerged(result) : 
            ds_time = pending [(- 1)] [0] 
            ds_values = pending [(- 1)] [1] 
            ds_names = result [1].keys () 
            return (ds_time, OrderedDict (zip (ds_names, ds_values))) 
deferred = threads.deferToThread (doinfo) 
        if pending : 
            deferred.addCallback (returnmerged) 
return deferred 
if (not defer) : 
        return doinfo () 
else : 
        if self._client : 
            deferred = self.pending (filename) 
            deferred.addCallback (self) 
            return deferred 
else : 
            return threads.deferToThread (doinfo) 

    
    

    Fix the buggy line: deferred.addCallback (self)
assistant
#Here is Python code to fix the buggy line: deferred.

deferred.addCallback (def merge()