diff --git a/api/core/workflow/nodes/tool/tool_node.py b/api/core/workflow/nodes/tool/tool_node.py
index b03ad45e6ce8cd..ca217182ccfd5c 100644
--- a/api/core/workflow/nodes/tool/tool_node.py
+++ b/api/core/workflow/nodes/tool/tool_node.py
@@ -78,7 +78,7 @@ def _convert_tool_messages(self, messages: list[ToolInvokeMessage]) -> tuple[str
             messages=messages,
             user_id=self.user_id,
             tenant_id=self.tenant_id,
-            conversation_id='',
+            conversation_id=None,
         )
         # extract plain text and files
         files = self._extract_tool_response_binary(messages)