diff --git a/api/db/services/dialog_service.py b/api/db/services/dialog_service.py
index 7edc94b16c..ef5f9f00b3 100644
--- a/api/db/services/dialog_service.py
+++ b/api/db/services/dialog_service.py
@@ -764,6 +764,8 @@ async def async_chat(dialog, messages, stream=True, **kwargs):
     knowledge_text = "\n\n------\n\n".join(knowledges)
     if knowledge_text:
         kwargs["knowledge"] = "\n------\n" + knowledge_text
+    else:
+        kwargs.setdefault("knowledge", "")
     gen_conf = dialog.llm_setting
 
     system_content = prompt_config["system"].format(**kwargs) + attachments_