human
Solve the following python programming problem given the following code:
def defer(self, opres) : 
    '
        Converts a raw :class:`couchbase.results.AsyncResult` object
        into a :class:`Deferred`.

        This is shorthand for the following "non-idiom"::

          d = Deferred()
          opres = cb.upsert("foo", "bar")
          opres.callback = d.callback

          def d_err(res, ex_type, ex_val, ex_tb):
              d.errback(opres, ex_type, ex_val, ex_tb)

          opres.errback = d_err
          return d

        :param opres: The operation to wrap
        :type opres: :class:`couchbase.results.AsyncResult`

        :return: a :class:`Deferred` object.

        Example::

          opres = cb.upsert("foo", "bar")
          d = cb.defer(opres)
          def on_ok(res):
              print("Result OK. Cas: {0}".format(res.cas))
          d.addCallback(opres)

        ' 
    d = Deferred () 
    opres.callback = opres.callback 
    def _on_err(mres, ex_type, ex_val, ex_tb) : 
        try : 
            raise ex_type (ex_val) 
except CouchbaseError : 
            d.errback () 
opres.errback = _on_err 
    return d 

    
    

    Fix the buggy line: opres.callback = opres.callback
assistant
#Here is Python program to fix the buggy line: opres.

opres.callback = d.callback