diff --git a/federationsender/internal/perform.go b/federationsender/internal/perform.go
index d014ca5c7..bff6833ca 100644
--- a/federationsender/internal/perform.go
+++ b/federationsender/internal/perform.go
@@ -2,6 +2,7 @@ package internal
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"time"
@@ -174,8 +175,10 @@ func (r *FederationSenderInternalAPI) performJoinUsingServer(
 
 	// Work out if we support the room version that has been supplied in
 	// the make_join response.
+	// "If not provided, the room version is assumed to be either "1" or "2"."
+	// https://matrix.org/docs/spec/server_server/unstable#get-matrix-federation-v1-make-join-roomid-userid
 	if respMakeJoin.RoomVersion == "" {
-		respMakeJoin.RoomVersion = gomatrixserverlib.RoomVersionV1
+		respMakeJoin.RoomVersion = setDefaultRoomVersionFromJoinEvent(respMakeJoin.JoinEvent)
 	}
 	if _, err = respMakeJoin.RoomVersion.EventFormat(); err != nil {
 		return fmt.Errorf("respMakeJoin.RoomVersion.EventFormat: %w", err)
@@ -205,6 +208,9 @@ func (r *FederationSenderInternalAPI) performJoinUsingServer(
 		return fmt.Errorf("r.federation.SendJoin: %w", err)
 	}
 	r.statistics.ForServer(serverName).Success()
+	if err := sanityCheckSendJoinResponse(respSendJoin); err != nil {
+		return err
+	}
 
 	// Process the join response in a goroutine. The idea here is
 	// that we'll try and wait for as long as possible for the work
@@ -424,3 +430,53 @@ func (r *FederationSenderInternalAPI) PerformBroadcastEDU(
 
 	return nil
 }
+
+func sanityCheckSendJoinResponse(respSendJoin gomatrixserverlib.RespSendJoin) error {
+	// sanity check we have a create event and it has a known room version
+	for _, ev := range respSendJoin.AuthEvents {
+		if ev.Type() == gomatrixserverlib.MRoomCreate && ev.StateKeyEquals("") {
+			// make sure the room version is known
+			content := ev.Content()
+			verBody := struct {
+				Version string `json:"room_version"`
+			}{}
+			err := json.Unmarshal(content, &verBody)
+			if err != nil {
+				return err
+			}
+			if verBody.Version == "" {
+				// https://matrix.org/docs/spec/client_server/r0.6.0#m-room-create
+				// The version of the room. Defaults to "1" if the key does not exist.
+				verBody.Version = "1"
+			}
+			knownVersions := gomatrixserverlib.RoomVersions()
+			if _, ok := knownVersions[gomatrixserverlib.RoomVersion(verBody.Version)]; !ok {
+				return fmt.Errorf("send_join m.room.create event has an unknown room version: %s", verBody.Version)
+			}
+			return nil
+		}
+	}
+	return fmt.Errorf("send_join response is missing m.room.create event")
+}
+
+func setDefaultRoomVersionFromJoinEvent(joinEvent gomatrixserverlib.EventBuilder) gomatrixserverlib.RoomVersion {
+	// if auth events are not event references we know it must be v3+
+	// we have to do these shenanigans to satisfy sytest, specifically for:
+	// "Outbound federation rejects m.room.create events with an unknown room version"
+	hasEventRefs := true
+	authEvents, ok := joinEvent.AuthEvents.([]interface{})
+	if ok {
+		if len(authEvents) > 0 {
+			_, ok = authEvents[0].(string)
+			if ok {
+				// event refs are objects, not strings, so we know we must be dealing with a v3+ room.
+				hasEventRefs = false
+			}
+		}
+	}
+
+	if hasEventRefs {
+		return gomatrixserverlib.RoomVersionV1
+	}
+	return gomatrixserverlib.RoomVersionV4
+}
diff --git a/sytest-whitelist b/sytest-whitelist
index 099fc6cbd..805f0e4dd 100644
--- a/sytest-whitelist
+++ b/sytest-whitelist
@@ -68,6 +68,8 @@ Request to logout with invalid an access token is rejected
 Request to logout without an access token is rejected
 Room creation reports m.room.create to myself
 Room creation reports m.room.member to myself
+Outbound federation rejects send_join responses with no m.room.create event
+Outbound federation rejects m.room.create events with an unknown room version
 Invited user can see room metadata
 # Blacklisted because these tests call /r0/events which we don't implement
 # New room members see their own join event