diff --git a/tests/federation_room_join_partial_state_test.go b/tests/federation_room_join_partial_state_test.go
index 72693e05..3210c820 100644
--- a/tests/federation_room_join_partial_state_test.go
+++ b/tests/federation_room_join_partial_state_test.go
@@ -20,6 +20,7 @@ import (
 	"github.com/gorilla/mux"
 	"github.com/tidwall/gjson"
 
+	"github.com/matrix-org/gomatrix"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/util"
 
@@ -489,6 +490,51 @@ func testReceiveEventDuringPartialStateJoin(
 		t.Fatalf("GET /event failed with %d: %s", res.StatusCode, string(eventResBody))
 	}
 
+	// fire off a /state_ids request for the last event.
+	// it must either:
+	//   * block because the homeserver does not have full state at the last event
+	//   * or 403 because the homeserver does not have full state yet and does not consider the
+	//     Complement homeserver to be in the room
+	stateIdsResponseChan := make(chan *gomatrixserverlib.RespStateIDs)
+	defer close(stateIdsResponseChan)
+	go func() {
+		stateReq := gomatrixserverlib.NewFederationRequest("GET", "hs1",
+			fmt.Sprintf("/_matrix/federation/v1/state_ids/%s?event_id=%s",
+				url.PathEscape(psjResult.ServerRoom.RoomID),
+				url.QueryEscape(event.EventID()),
+			),
+		)
+		var respStateIDs gomatrixserverlib.RespStateIDs
+		if err := psjResult.Server.SendFederationRequest(deployment, stateReq, &respStateIDs); err != nil {
+			httpErr, ok := err.(gomatrix.HTTPError)
+			t.Logf("%v", httpErr)
+			if ok && httpErr.Code == 403 {
+				stateIdsResponseChan <- nil
+				return
+			}
+			t.Errorf("/state_ids request returned non-200: %s", err)
+			return
+		}
+		stateIdsResponseChan <- &respStateIDs
+	}()
+
+	select {
+	case <-time.After(1 * time.Second):
+		t.Logf("/state_ids request for event %s blocked as expected", event.EventID())
+		defer func() { <-stateIdsResponseChan }()
+		break
+	case respStateIDs := <-stateIdsResponseChan:
+		if respStateIDs == nil {
+			t.Logf("/state_ids request for event %s returned 403 as expected", event.EventID())
+		} else {
+			// since we have not yet given the homeserver the full state at the join event and allowed
+			// the partial join to complete, it can't possibly know the full state at the last event.
+			// While it may be possible for the response to be correct by some accident of state res,
+			// the homeserver is still wrong in spirit.
+			t.Fatalf("/state_ids request for event %s did not block when it should have", event.EventID())
+		}
+	}
+
 	// allow the partial join to complete
 	psjResult.FinishStateRequest()
 	alice.MustSyncUntil(t,