diff --git a/federationapi/routing/publicrooms.go b/federationapi/routing/publicrooms.go
index ddd92c5c0..5b9be8807 100644
--- a/federationapi/routing/publicrooms.go
+++ b/federationapi/routing/publicrooms.go
@@ -156,7 +156,9 @@ func fillInRooms(ctx context.Context, roomIDs []string, rsAPI roomserverAPI.Room
 			case topicTuple:
 				pub.Topic = contentVal
 			case canonicalTuple:
-				pub.CanonicalAlias = contentVal
+				if _, _, err := gomatrixserverlib.SplitID('#', contentVal); err == nil {
+					pub.CanonicalAlias = contentVal
+				}
 			case visibilityTuple:
 				pub.WorldReadable = contentVal == "world_readable"
 			// need both of these to determine whether guests can join
diff --git a/roomserver/api/wrapper.go b/roomserver/api/wrapper.go
index 2ebe2f648..de66df803 100644
--- a/roomserver/api/wrapper.go
+++ b/roomserver/api/wrapper.go
@@ -215,7 +215,9 @@ func PopulatePublicRooms(ctx context.Context, roomIDs []string, rsAPI Roomserver
 			case topicTuple:
 				pub.Topic = contentVal
 			case canonicalTuple:
-				pub.CanonicalAlias = contentVal
+				if _, _, err := gomatrixserverlib.SplitID('#', contentVal); err == nil {
+					pub.CanonicalAlias = contentVal
+				}
 			case visibilityTuple:
 				pub.WorldReadable = contentVal == "world_readable"
 			// need both of these to determine whether guests can join