diff --git a/swh/journal/pytest_plugin.py b/swh/journal/pytest_plugin.py
--- a/swh/journal/pytest_plugin.py
+++ b/swh/journal/pytest_plugin.py
@@ -83,10 +83,14 @@
                 del value["ctime"]
 
         for key in known_keys:
-            assert key in received_keys, (
-                f"expected {object_type} key {hash_to_hex(key)} "
-                "absent from consumed messages"
-            )
+            try:
+                key_str = hash_to_hex(key)
+            except TypeError:
+                key_str = repr(key)
+
+            assert (
+                key in received_keys
+            ), f"expected {object_type} key {key_str} absent from consumed messages"
 
         if exclude and object_type in exclude:
             continue