diff --git a/tests/test_grpc.py b/tests/test_grpc.py
index 15462cfb6..3a3ef849c 100644
--- a/tests/test_grpc.py
+++ b/tests/test_grpc.py
@@ -261,8 +261,9 @@ def test_message_loop(mock_heartbeat_request, _mock_sleep, _mock_event):
     channel = mock.MagicMock()
     terminate_event = threading.Event()
     state_record = StateRecord()
+    participant_id = "123"
 
-    message_loop(channel, state_record, terminate_event)
+    message_loop(channel, participant_id, state_record, terminate_event)
 
     # check that the heartbeat is sent exactly twice
     expected_call = mock.call(round=-1, state=State.READY)
@@ -293,9 +294,9 @@ def test_start_training_round(coordinator_service):
     # simulate a participant communicating with coordinator via channel
     with grpc.insecure_channel("localhost:50051") as channel:
         # we need to rendezvous before we can send any other requests
-        rendezvous(channel)
+        rendezvous(channel, participant_id="123")
         # call StartTrainingRound service method on coordinator
-        epochs, epoch_base = start_training_round(channel)
+        epochs, epoch_base = start_training_round(channel, participant_id="123")
 
     # check global model received
     assert epochs == 5
@@ -364,7 +365,7 @@ def test_end_training_round(
 
     with grpc.insecure_channel("localhost:50051") as channel:
         # we first need to rendezvous before we can send any other request
-        rendezvous(channel)
+        rendezvous(channel, participant_id="123")
         # call EndTrainingRound service method on coordinator
         participant_store.write_weights("participant1", 0, test_weights)
         end_training_round(
diff --git a/xain_fl/coordinator/coordinator.py b/xain_fl/coordinator/coordinator.py
index e3ab8a042..274181d02 100644
--- a/xain_fl/coordinator/coordinator.py
+++ b/xain_fl/coordinator/coordinator.py
@@ -275,7 +275,8 @@ def select_outstanding(self) -> List[str]:
         frac = num_outstanding / len(pool)
 
         self.controller.fraction_of_participants = frac
-        return self.controller.select_ids(list(pool))
+        outstanding: List[str] = self.controller.select_ids(list(pool))
+        return outstanding
 
     def _handle_rendezvous(
         self, _message: RendezvousRequest, participant_id: str