diff --git a/changelog.d/11278.misc b/changelog.d/11278.misc
new file mode 100644
index 0000000000..9b014bc8b4
--- /dev/null
+++ b/changelog.d/11278.misc
@@ -0,0 +1 @@
+Fix a small typo in the error response when a relation type other than 'm.annotation' is passed to `GET /rooms/{room_id}/aggregations/{event_id}`.
\ No newline at end of file
diff --git a/synapse/rest/client/relations.py b/synapse/rest/client/relations.py
index 58f6699073..184cfbe196 100644
--- a/synapse/rest/client/relations.py
+++ b/synapse/rest/client/relations.py
@@ -298,7 +298,9 @@ class RelationAggregationPaginationServlet(RestServlet):
             raise SynapseError(404, "Unknown parent event.")
 
         if relation_type not in (RelationTypes.ANNOTATION, None):
-            raise SynapseError(400, "Relation type must be 'annotation'")
+            raise SynapseError(
+                400, f"Relation type must be '{RelationTypes.ANNOTATION}'"
+            )
 
         limit = parse_integer(request, "limit", default=5)
         from_token_str = parse_string(request, "from")