diff --git a/CONTRIBUTORS b/CONTRIBUTORS
new file mode 100644
--- /dev/null
+++ b/CONTRIBUTORS
@@ -0,0 +1 @@
+Kumar Shivendu
diff --git a/swh/journal/tests/test_inmemory.py b/swh/journal/tests/test_inmemory.py
new file mode 100644
--- /dev/null
+++ b/swh/journal/tests/test_inmemory.py
@@ -0,0 +1,14 @@
+import pytest
+
+from swh.journal.writer import model_object_dict_sanitizer
+from swh.journal.writer.inmemory import InMemoryJournalWriter
+from swh.model.model import BaseModel
+
+
+def test_write_addition_errors_without_unique_key():
+    writer = InMemoryJournalWriter[BaseModel](
+        value_sanitizer=model_object_dict_sanitizer
+    )
+
+    with pytest.raises(NotImplementedError):
+        writer.write_addition("BaseModel", BaseModel())
diff --git a/swh/journal/tests/test_kafka_writer.py b/swh/journal/tests/test_kafka_writer.py
--- a/swh/journal/tests/test_kafka_writer.py
+++ b/swh/journal/tests/test_kafka_writer.py
@@ -234,3 +234,15 @@
 
     with pytest.raises(KafkaDeliveryError):
         writer.write_addition("directory", empty_dir)
+
+
+def test_write_addition_errors_without_unique_key(kafka_prefix: str, kafka_server: str):
+    writer = KafkaJournalWriter[BaseModel](
+        brokers=[kafka_server],
+        client_id="kafka_writer",
+        prefix=kafka_prefix,
+        value_sanitizer=model_object_dict_sanitizer,
+    )
+
+    with pytest.raises(NotImplementedError):
+        writer.write_addition("BaseModel", BaseModel())