diff --git a/src/sw.ts b/src/sw.ts
index 2179dfc..cb84e3a 100644
--- a/src/sw.ts
+++ b/src/sw.ts
@@ -3,15 +3,30 @@
 export type {};
 declare const self: ServiceWorkerGlobalScope;
 
-async function askForAccessToken(client: Client): Promise<string | undefined> {
+/**
+ * Pending token requests keyed by responseKey
+ */
+const pendingTokenRequests = new Map<string, (token: string | undefined) => void>();
+
+/**
+ * Message handler must be registered at top level for service worker compliance
+ */
+self.addEventListener('message', (event: ExtendableMessageEvent) => {
+  const { responseKey, token } = event.data;
+  if (responseKey && pendingTokenRequests.has(responseKey)) {
+    const resolve = pendingTokenRequests.get(responseKey)!;
+    pendingTokenRequests.delete(responseKey);
+    resolve(token);
+  }
+});
+
+/**
+ * Request an access token from the client
+ */
+function askForAccessToken(client: Client): Promise<string | undefined> {
   return new Promise((resolve) => {
     const responseKey = Math.random().toString(36);
-    const listener = (event: ExtendableMessageEvent) => {
-      if (event.data.responseKey !== responseKey) return;
-      resolve(event.data.token);
-      self.removeEventListener('message', listener);
-    };
-    self.addEventListener('message', listener);
+    pendingTokenRequests.set(responseKey, resolve);
     client.postMessage({ responseKey, type: 'token' });
   });
 }