diff --git a/changelog.d/12823.bugfix b/changelog.d/12823.bugfix
new file mode 100644
index 0000000000..1a1f5957e7
--- /dev/null
+++ b/changelog.d/12823.bugfix
@@ -0,0 +1 @@
+Fix a bug, introduced in Synapse 1.21.0, that led to media thumbnails being unusable before the index has been added in the background.
diff --git a/synapse/storage/database.py b/synapse/storage/database.py
index 5ddb58a8a2..a78d68a9d7 100644
--- a/synapse/storage/database.py
+++ b/synapse/storage/database.py
@@ -90,6 +90,8 @@ UNIQUE_INDEX_BACKGROUND_UPDATES = {
     "device_lists_remote_extremeties": "device_lists_remote_extremeties_unique_idx",
     "device_lists_remote_cache": "device_lists_remote_cache_unique_idx",
     "event_search": "event_search_event_id_idx",
+    "local_media_repository_thumbnails": "local_media_repository_thumbnails_method_idx",
+    "remote_media_cache_thumbnails": "remote_media_repository_thumbnails_method_idx",
 }