diff --git a/api/core/tools/custom_tool/tool.py b/api/core/tools/custom_tool/tool.py
index 2f2f1ebbdd..2f5cc6d4c0 100644
--- a/api/core/tools/custom_tool/tool.py
+++ b/api/core/tools/custom_tool/tool.py
@@ -168,7 +168,7 @@ class ApiTool(Tool):
                 cookies[parameter["name"]] = value
 
             elif parameter["in"] == "header":
-                headers[parameter["name"]] = value
+                headers[parameter["name"]] = str(value)
 
         # check if there is a request body and handle it
         if "requestBody" in self.api_bundle.openapi and self.api_bundle.openapi["requestBody"] is not None: