diff --git a/federationapi/routing/join.go b/federationapi/routing/join.go
index 1ef1fe40e..0fdaeb1fe 100644
--- a/federationapi/routing/join.go
+++ b/federationapi/routing/join.go
@@ -229,6 +229,21 @@ func SendJoin(
 		}
 	}
 
+	// Check that this is in fact a join event
+	membership, err := event.Membership()
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: jsonerror.BadJSON("missing content.membership key"),
+		}
+	}
+	if membership != "join" {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: jsonerror.BadJSON("membership must be 'join'"),
+		}
+	}
+
 	// Check that the event is signed by the server sending the request.
 	redacted := event.Redact()
 	verifyRequests := []gomatrixserverlib.VerifyJSONRequest{{
diff --git a/federationapi/routing/leave.go b/federationapi/routing/leave.go
index 812cf947e..38f4ca76f 100644
--- a/federationapi/routing/leave.go
+++ b/federationapi/routing/leave.go
@@ -248,7 +248,10 @@ func SendLeave(
 	mem, err := event.Membership()
 	if err != nil {
 		util.GetLogger(httpReq.Context()).WithError(err).Error("event.Membership failed")
-		return jsonerror.InternalServerError()
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: jsonerror.BadJSON("missing content.membership key"),
+		}
 	}
 	if mem != gomatrixserverlib.Leave {
 		return util.JSONResponse{