diff --git a/clientapi/jsonerror/jsonerror.go b/clientapi/jsonerror/jsonerror.go
index 735de5bea..85e887aec 100644
--- a/clientapi/jsonerror/jsonerror.go
+++ b/clientapi/jsonerror/jsonerror.go
@@ -18,6 +18,7 @@ import (
 	"fmt"
 	"net/http"
 
+	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/util"
 )
 
@@ -124,6 +125,12 @@ func GuestAccessForbidden(msg string) *MatrixError {
 	return &MatrixError{"M_GUEST_ACCESS_FORBIDDEN", msg}
 }
 
+// IncompatibleRoomVersion is an error which is returned when the client
+// requests a room with a version that is unsupported.
+func IncompatibleRoomVersion(roomVersion gomatrixserverlib.RoomVersion) *MatrixError {
+	return &MatrixError{"M_INCOMPATIBLE_ROOM_VERSION", string(roomVersion)}
+}
+
 // UnsupportedRoomVersion is an error which is returned when the client
 // requests a room with a version that is unsupported.
 func UnsupportedRoomVersion(msg string) *MatrixError {
diff --git a/federationapi/routing/join.go b/federationapi/routing/join.go
index 23f39ea46..e851cb086 100644
--- a/federationapi/routing/join.go
+++ b/federationapi/routing/join.go
@@ -62,9 +62,7 @@ func MakeJoin(
 	if !remoteSupportsVersion {
 		return util.JSONResponse{
 			Code: http.StatusBadRequest,
-			JSON: jsonerror.UnsupportedRoomVersion(
-				fmt.Sprintf("Joining server does not support room version %s", verRes.RoomVersion),
-			),
+			JSON: jsonerror.IncompatibleRoomVersion(verRes.RoomVersion),
 		}
 	}