diff --git a/packages-exp/auth-exp/src/core/auth/auth_impl.ts b/packages-exp/auth-exp/src/core/auth/auth_impl.ts
index 357987f348d..fad70bd6033 100644
--- a/packages-exp/auth-exp/src/core/auth/auth_impl.ts
+++ b/packages-exp/auth-exp/src/core/auth/auth_impl.ts
@@ -82,8 +82,9 @@ export class AuthImpl implements Auth, _FirebaseService {
   readonly name: string;
 
   // Tracks the last notified UID for state change listeners to prevent
-  // repeated calls to the callbacks
-  private lastNotifiedUid: string | undefined = undefined;
+  // repeated calls to the callbacks. Undefined means it's never been
+  // called, whereas null means it's been called with a signed out user
+  private lastNotifiedUid: string | null | undefined = undefined;
 
   languageCode: string | null = null;
   tenantId: string | null = null;
@@ -474,8 +475,9 @@ export class AuthImpl implements Auth, _FirebaseService {
 
     this.idTokenSubscription.next(this.currentUser);
 
-    if (this.lastNotifiedUid !== this.currentUser?.uid) {
-      this.lastNotifiedUid = this.currentUser?.uid;
+    const currentUid = this.currentUser?.uid ?? null;
+    if (this.lastNotifiedUid !== currentUid) {
+      this.lastNotifiedUid = currentUid;
       this.authStateSubscription.next(this.currentUser);
     }
   }
diff --git a/packages-exp/auth-types-exp/index.d.ts b/packages-exp/auth-types-exp/index.d.ts
index c873605e54f..da6a2cbd8de 100644
--- a/packages-exp/auth-types-exp/index.d.ts
+++ b/packages-exp/auth-types-exp/index.d.ts
@@ -267,7 +267,7 @@ export interface Auth {
    * @param completed - callback triggered when observer is removed.
    */
   onAuthStateChanged(
-    nextOrObserver: NextOrObserver<User>,
+    nextOrObserver: NextOrObserver<User | null>,
     error?: ErrorFn,
     completed?: CompleteFn
   ): Unsubscribe;
@@ -282,7 +282,7 @@ export interface Auth {
    * @param completed - callback triggered when observer is removed.
    */
   onIdTokenChanged(
-    nextOrObserver: NextOrObserver<User>,
+    nextOrObserver: NextOrObserver<User | null>,
     error?: ErrorFn,
     completed?: CompleteFn
   ): Unsubscribe;