diff --git a/federation/server_room.go b/federation/server_room.go
index 61c40ed3..25cc18d8 100644
--- a/federation/server_room.go
+++ b/federation/server_room.go
@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"log"
+	"strconv"
 	"sync"
 	"time"
 
@@ -333,15 +334,22 @@ func InitialRoomEvents(roomVer gomatrixserverlib.RoomVersion, creator string) []
 			Type:     "m.room.create",
 			StateKey: b.Ptr(""),
 			Sender:   creator,
-			Content: map[string]interface{}{
-				"creator":      creator,
-				"room_version": roomVer,
-				// We have to add randomness to the create event, else if you create 2x v12+ rooms in the same millisecond
-				// they will get the same room ID, clobbering internal data structures and causing extremely confusing
-				// behaviour. By adding this entropy, we ensure that even if rooms are created in the same millisecond, their
-				// hashes will not be the same.
-				"complement_entropy": util.RandomString(18),
-			},
+			Content: func() map[string]interface{} {
+				content := map[string]interface{}{
+					"room_version": roomVer,
+					// We have to add randomness to the create event, else if you create 2x v12+ rooms in the same millisecond
+					// they will get the same room ID, clobbering internal data structures and causing extremely confusing
+					// behaviour. By adding this entropy, we ensure that even if rooms are created in the same millisecond, their
+					// hashes will not be the same.
+					"complement_entropy": util.RandomString(18),
+				}
+				// The creator field was removed in room version 11 (MSC4239).
+				n, err := strconv.Atoi(string(roomVer))
+				if err != nil || n < 11 {
+					content["creator"] = creator
+				}
+				return content
+			}(),
 		},
 		{
 			Type:     "m.room.member",
diff --git a/tests/csapi/rooms_state_test.go b/tests/csapi/rooms_state_test.go
index 2158cae1..4e56f6e9 100644
--- a/tests/csapi/rooms_state_test.go
+++ b/tests/csapi/rooms_state_test.go
@@ -4,6 +4,7 @@
 package csapi_tests
 
 import (
+	"strconv"
 	"testing"
 	"time"
 
@@ -26,6 +27,8 @@ func TestRoomCreationReportsEventsToMyself(t *testing.T) {
 		LocalpartSuffix: "bob",
 		Password:        "bobpassword",
 	})
+	defaultVer := alice.GetDefaultRoomVersion(t)
+	defaultVerN, _ := strconv.Atoi(string(defaultVer))
 	roomID := alice.MustCreateRoom(t, map[string]interface{}{})
 
 	t.Run("parallel", func(t *testing.T) {
@@ -38,7 +41,10 @@ func TestRoomCreationReportsEventsToMyself(t *testing.T) {
 					return false
 				}
 				must.Equal(t, ev.Get("sender").Str, alice.UserID, "wrong sender")
-				must.Equal(t, ev.Get("content").Get("creator").Str, alice.UserID, "wrong content.creator")
+				// The creator field was removed in room version 11 (MSC4239).
+				if defaultVerN < 11 {
+					must.Equal(t, ev.Get("content").Get("creator").Str, alice.UserID, "wrong content.creator")
+				}
 				return true
 			}))
 		})
diff --git a/tests/federation_room_get_missing_events_test.go b/tests/federation_room_get_missing_events_test.go
index 9899f88b..541bd0ef 100644
--- a/tests/federation_room_get_missing_events_test.go
+++ b/tests/federation_room_get_missing_events_test.go
@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"net/http"
+	"strconv"
 	"strings"
 	"testing"
 	"time"
@@ -497,6 +498,22 @@ func TestOutboundFederationEventSizeGetMissingEvents(t *testing.T) {
 	}).Methods("POST")
 
 	ver := alice.GetDefaultRoomVersion(t)
+	// This test crafts a "bad" event which state_key is 280 bytes but only 70
+	// codepoints.
+	//
+	// Room version 11 in Synapse switched from using codepoints to using
+	// bytes. Which means the 280-byte state_key would be rejected immediately.
+	// Use room version 10 in that case so the codepoint-based limit is in effect.
+	//
+	// Since upgrading a room (for example from v10 to v11) won't carry the event
+	// (a new room is created), we don't have to worry about v10 room events in a v11
+	// room.
+	//
+	// So this test is essentially skipped for any default room v11 or higher.
+	verNum, _ := strconv.Atoi(string(ver))
+	if verNum >= 11 {
+		ver = gomatrixserverlib.RoomVersion("10")
+	}
 	charlie := srv.UserID("charlie")
 	room := srv.MustMakeRoom(t, ver, federation.InitialRoomEvents(ver, charlie))
 	roomAlias := srv.MakeAliasMapping("flibble", room.RoomID)