diff --git a/types/amplitude-js/amplitude-js-tests.ts b/types/amplitude-js/amplitude-js-tests.ts
index 87e32e5d21..f5eb4b30b9 100644
--- a/types/amplitude-js/amplitude-js-tests.ts
+++ b/types/amplitude-js/amplitude-js-tests.ts
@@ -68,6 +68,7 @@ module Amplitude.Tests {
         client.setDeviceId('45f0954f-eb79-4463-ac8a-233a6f45a8f0');
         client.setDomain('.amplitude.com');
         client.setUserId('joe@gmail.com');
+        client.setUserId(null);
         client.setOptOut(true);
         client.setGroup('type', 'name');
         client.setGroup('type', ['name', 'name2']);
diff --git a/types/amplitude-js/index.d.ts b/types/amplitude-js/index.d.ts
index c6773cdad2..7558c732d1 100644
--- a/types/amplitude-js/index.d.ts
+++ b/types/amplitude-js/index.d.ts
@@ -122,7 +122,7 @@ export function getSessionId(): number;
 
 export function setDomain(domain: string): void;
 
-export function setUserId(userId: string): void;
+export function setUserId(userId: string | null): void;
 
 export function setDeviceId(id: string): void;
 export function regenerateDeviceId(): void;