diff --git a/swh/indexer/storage/in_memory.py b/swh/indexer/storage/in_memory.py
--- a/swh/indexer/storage/in_memory.py
+++ b/swh/indexer/storage/in_memory.py
@@ -309,6 +309,8 @@
                 default)
 
         """
+        if not all(isinstance(x['id'], bytes) for x in languages):
+            raise TypeError('identifiers must be bytes.')
         self._languages.add(languages, conflict_update)
 
     def content_ctags_missing(self, ctags):