diff --git a/changelog.d/14537.bugfix b/changelog.d/14537.bugfix
new file mode 100644
index 000000000..d7ce78d03
--- /dev/null
+++ b/changelog.d/14537.bugfix
@@ -0,0 +1 @@
+Fix a long-standing bug where the [List media admin API](https://matrix-org.github.io/synapse/latest/admin_api/media_admin_api.html#list-all-media-in-a-room) would fail when processing an image with broken thumbnail information.
\ No newline at end of file
diff --git a/synapse/storage/databases/main/room.py b/synapse/storage/databases/main/room.py
index 4fbaefad7..52ad947c6 100644
--- a/synapse/storage/databases/main/room.py
+++ b/synapse/storage/databases/main/room.py
@@ -912,7 +912,11 @@ class RoomWorkerStore(CacheInvalidationWorkerStore):
                 event_json = db_to_json(content_json)
                 content = event_json["content"]
                 content_url = content.get("url")
-                thumbnail_url = content.get("info", {}).get("thumbnail_url")
+                info = content.get("info")
+                if isinstance(info, dict):
+                    thumbnail_url = info.get("thumbnail_url")
+                else:
+                    thumbnail_url = None
 
                 for url in (content_url, thumbnail_url):
                     if not url: