diff --git a/finbot/agents/chat.py b/finbot/agents/chat.py
index ba2a0bd7..3574288c 100644
--- a/finbot/agents/chat.py
+++ b/finbot/agents/chat.py
@@ -190,6 +190,9 @@ async def _call_start_workflow(
         if not self.background_tasks:
             return json.dumps({"error": "Workflow engine not available"})
 
+        if vendor_id is None:
+            return json.dumps({"error": "vendor_id is required"})
+
         from finbot.agents.runner import (
             run_orchestrator_agent,  # pylint: disable=import-outside-toplevel
         )