diff --git a/frontend/src/components/workflows/WorkflowEditor.tsx b/frontend/src/components/workflows/WorkflowEditor.tsx
index e52bfc9..b64838f 100644
--- a/frontend/src/components/workflows/WorkflowEditor.tsx
+++ b/frontend/src/components/workflows/WorkflowEditor.tsx
@@ -169,7 +169,7 @@ export function WorkflowEditor({ open, workflow, onClose }: WorkflowEditorProps)
       // Show detailed validation errors from backend (422)
       if (err.validationErrors) {
         const details = Object.entries(err.validationErrors)
-          .map(([field, msgs]) => `${field}: ${msgs.join(', ')}`)
+          .map(([field, msgs]) => `${field}: ${(msgs as string[]).join(', ')}`)
           .join('; ');
         toast.error(`Validierungsfehler: ${details}`);
       } else if (err.detail) {