diff --git a/openapi/components/schemas/GroupMember.yaml b/openapi/components/schemas/GroupMember.yaml
index ff617cca..d97d3df4 100644
--- a/openapi/components/schemas/GroupMember.yaml
+++ b/openapi/components/schemas/GroupMember.yaml
@@ -26,6 +26,7 @@ properties:
   joinedAt:
     type: string
     format: date-time
+    nullable: true
   membershipStatus:
     type: string
     example: member