diff --git a/src/trace/interceptors/fetch.ts b/src/trace/interceptors/fetch.ts
index 1c97835..f62f975 100644
--- a/src/trace/interceptors/fetch.ts
+++ b/src/trace/interceptors/fetch.ts
@@ -83,9 +83,13 @@ export default function windowFetch(options: CustomOptionsType, segments: Segmen
         args[1] = {};
       }
       if (!args[1].headers) {
-        args[1].headers = {};
+        args[1].headers = new Headers();
+      }
+      if (args[1].headers instanceof Headers) {
+        args[1].headers.append('sw8', values);
+      } else {
+        args[1].headers['sw8'] = values;
       }
-      args[1].headers['sw8'] = values;
     }
 
     const response = await originFetch(...args);