diff --git a/lib/modules/messaging/index.js b/lib/modules/messaging/index.js
index cf163087..52aad936 100644
--- a/lib/modules/messaging/index.js
+++ b/lib/modules/messaging/index.js
@@ -20,7 +20,7 @@ type OnMessageObserver = {
   next: OnMessage,
 };
 
-type OnTokenRefresh = String => any;
+type OnTokenRefresh = string => any;
 
 type OnTokenRefreshObserver = {
   next: OnTokenRefresh,
@@ -101,7 +101,7 @@ export default class Messaging extends ModuleBase {
   onTokenRefresh(
     nextOrObserver: OnTokenRefresh | OnTokenRefreshObserver
   ): () => any {
-    let listener: String => any;
+    let listener: string => any;
     if (isFunction(nextOrObserver)) {
       // $FlowExpectedError: Not coping with the overloaded method signature
       listener = nextOrObserver;