diff --git a/synapse/storage/databases/main/events.py b/synapse/storage/databases/main/events.py
index 586f488f25..b043df68f5 100644
--- a/synapse/storage/databases/main/events.py
+++ b/synapse/storage/databases/main/events.py
@@ -3106,7 +3106,7 @@ class PersistEventsStore:
                 txn,
                 event,
                 "content.topic",
-                get_plain_text_topic_from_event_content(event.content),
+                get_plain_text_topic_from_event_content(event.content) or "",
             )
 
     def _store_room_name_txn(self, txn: LoggingTransaction, event: EventBase) -> None:
diff --git a/synapse/util/events.py b/synapse/util/events.py
index 4c609ad882..1e8f53c4e9 100644
--- a/synapse/util/events.py
+++ b/synapse/util/events.py
@@ -13,6 +13,8 @@
 #
 #
 
+from typing import Optional
+
 from synapse.types import JsonDict
 from synapse.util.stringutils import random_string
 
@@ -30,7 +32,7 @@ def generate_fake_event_id() -> str:
     return "$" + random_string(43)
 
 
-def get_plain_text_topic_from_event_content(content: JsonDict):
+def get_plain_text_topic_from_event_content(content: JsonDict) -> Optional[str]:
     """
     Given the content of an m.room.topic event returns the plain text topic
     representation if any exists.