diff --git a/src/client_server/membership.rs b/src/client_server/membership.rs
index 3876246..149df93 100644
--- a/src/client_server/membership.rs
+++ b/src/client_server/membership.rs
@@ -595,7 +595,8 @@ async fn join_room_by_id_helper(
             db.rooms.add_pdu_outlier(&pdu)?;
             if let Some(state_key) = &pdu.state_key {
                 if pdu.kind == EventType::RoomMember {
-                    let target_user_id = UserId::try_from(state_key.clone()).map_err(|_| {
+                    let target_user_id = UserId::try_from(state_key.clone()).map_err(|e| {
+                        warn!("Invalid user id in send_join response: {}: {}", state_key, e);
                         Error::BadServerResponse("Invalid user id in send_join response.")
                     })?;