diff --git a/roomserver/internal/helpers/auth.go b/roomserver/internal/helpers/auth.go
index a93368701..e9a57d8da 100644
--- a/roomserver/internal/helpers/auth.go
+++ b/roomserver/internal/helpers/auth.go
@@ -132,12 +132,18 @@ type authEvents struct {
 	events         EventMap
 }
 
+// Valid verifies that all auth events are from the same room.
 func (ae *authEvents) Valid() bool {
-	roomIDs := make(map[string]struct{})
-	for _, ev := range ae.events {
-		roomIDs[ev.RoomID()] = struct{}{}
+	roomID := ""
+	for i := range ae.events {
+		if i == 0 {
+			roomID = ae.events[i].RoomID()
+		}
+		if roomID != ae.events[i].RoomID() {
+			return false
+		}
 	}
-	return len(roomIDs) <= 1
+	return true
 }
 
 // Create implements gomatrixserverlib.AuthEventProvider
diff --git a/roomserver/internal/input/input_events_test.go b/roomserver/internal/input/input_events_test.go
index 60119b2ed..818e7715c 100644
--- a/roomserver/internal/input/input_events_test.go
+++ b/roomserver/internal/input/input_events_test.go
@@ -8,7 +8,7 @@ import (
 	"github.com/matrix-org/dendrite/test"
 )
 
-func Test_1(t *testing.T) {
+func Test_EventAuth(t *testing.T) {
 	alice := test.NewUser(t)
 	bob := test.NewUser(t)
 
@@ -35,7 +35,7 @@ func Test_1(t *testing.T) {
 		}
 	}
 
-	// Add the illegal auth event from room1
+	// Add the illegal auth event from room1 (rooms are different)
 	for _, x := range room1.Events() {
 		if x.Type() == gomatrixserverlib.MRoomMember {
 			authEventIDs = append(authEventIDs, x.EventID())
@@ -43,7 +43,7 @@ func Test_1(t *testing.T) {
 		}
 	}
 
-	// Craft the illegal join event
+	// Craft the illegal join event, with auth events from different rooms
 	ev := room2.CreateEvent(t, bob, "m.room.member", map[string]interface{}{
 		"membership": "join",
 	}, test.WithStateKey(bob.ID), test.WithAuthIDs(authEventIDs))