diff --git a/src/python/src/grpc/framework/face/_calls.py b/src/python/src/grpc/framework/face/_calls.py
index 75a550e3c7d739bef641e93ff031903a86f9e90b..87edeb0f0e511942ee2b679393215c5f67569730 100644
--- a/src/python/src/grpc/framework/face/_calls.py
+++ b/src/python/src/grpc/framework/face/_calls.py
@@ -248,7 +248,7 @@ class _OperationFuture(future.Future):
     """See future.Future.add_done_callback for specification."""
     with self._condition:
       if self._callbacks is not None:
-        self._callbacks.add(fn)
+        self._callbacks.append(fn)
         return
 
     callable_util.call_logging_exceptions(fn, _DONE_CALLBACK_LOG_MESSAGE, self)