diff --git a/finbot/agents/chat.py b/finbot/agents/chat.py
index ba2a0bd7..6be2c883 100644
--- a/finbot/agents/chat.py
+++ b/finbot/agents/chat.py
@@ -187,6 +187,9 @@ async def _call_start_workflow(
         invoice_id: int | None = None,
         attachment_file_ids: list[int] | None = None,
     ) -> str:
+        if description is None or vendor_id is None:
+            return json.dumps({"error": "description and vendor_id are required"})
+
         if not self.background_tasks:
             return json.dumps({"error": "Workflow engine not available"})