diff --git a/types/amplitude-js/amplitude-js-tests.ts b/types/amplitude-js/amplitude-js-tests.ts
index da6d30c548..87e32e5d21 100644
--- a/types/amplitude-js/amplitude-js-tests.ts
+++ b/types/amplitude-js/amplitude-js-tests.ts
@@ -36,6 +36,7 @@ module Amplitude.Tests {
         amplitude.setGroup('orgId', '15');
         amplitude.setGroup('orgId', ['15', '16']);
         amplitude.setUserId('joe@gmail.com');
+        amplitude.setUserId(null);
         amplitude.setUserProperties({ 'gender': 'female', 'sign_up_complete': true })
         amplitude.setVersionName('1.12.3');
         amplitude.isNewSession();
diff --git a/types/amplitude-js/index.d.ts b/types/amplitude-js/index.d.ts
index 58b25b1119..c6773cdad2 100644
--- a/types/amplitude-js/index.d.ts
+++ b/types/amplitude-js/index.d.ts
@@ -86,7 +86,7 @@ export class AmplitudeClient {
     getSessionId(): number;
 
     setDomain(domain: string): void;
-    setUserId(userId: string): void;
+    setUserId(userId: string | null): void;
 
     setDeviceId(id: string): void;
     regenerateDeviceId(): void;