diff --git a/Resources/Core/Functional/Extensions/json_response/Classes/Middleware/BackendUserHandler.php b/Resources/Core/Functional/Extensions/json_response/Classes/Middleware/BackendUserHandler.php
index 1b002c1d..81b6a782 100644
--- a/Resources/Core/Functional/Extensions/json_response/Classes/Middleware/BackendUserHandler.php
+++ b/Resources/Core/Functional/Extensions/json_response/Classes/Middleware/BackendUserHandler.php
@@ -55,6 +55,7 @@ public function process(ServerRequestInterface $request, RequestHandlerInterface
             // Init backend user if found in database
             $backendUser = GeneralUtility::makeInstance(FrontendBackendUserAuthentication::class);
             $backendUser->user = $row;
+            $workspaceId = $workspaceId ?? 0;
             if ($workspaceId !== null) {
                 // Force backend user into given workspace, can be 0, too.
                 $backendUser->setTemporaryWorkspace($workspaceId);