diff --git a/swh/journal/writer/inmemory.py b/swh/journal/writer/inmemory.py
index baecadc..4272834 100644
--- a/swh/journal/writer/inmemory.py
+++ b/swh/journal/writer/inmemory.py
@@ -1,42 +1,43 @@
 # Copyright (C) 2019 The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
 import logging
 from multiprocessing import Manager
 from typing import Any, Generic, List, Tuple, TypeVar
 
 from . import ValueProtocol
 
 logger = logging.getLogger(__name__)
 
 
 TValue = TypeVar("TValue", bound=ValueProtocol)
 
 
 class InMemoryJournalWriter(Generic[TValue]):
     objects: List[Tuple[str, TValue]]
     privileged_objects: List[Tuple[str, TValue]]
 
     def __init__(self, value_sanitizer: Any):
         # Share the list of objects across processes, for RemoteAPI tests.
         self.manager = Manager()
         self.objects = self.manager.list()
         self.privileged_objects = self.manager.list()
 
     def write_addition(
         self, object_type: str, object_: TValue, privileged: bool = False
     ) -> None:
+        object_.unique_key()  # Check this does not error, to mimic the kafka writer
         if privileged:
             self.privileged_objects.append((object_type, object_))
         else:
             self.objects.append((object_type, object_))
 
     write_update = write_addition
 
     def write_additions(
         self, object_type: str, objects: List[TValue], privileged: bool = False
     ) -> None:
         for object_ in objects:
             self.write_addition(object_type, object_, privileged)