diff --git a/services/core/java/com/android/server/am/UserController.java b/services/core/java/com/android/server/am/UserController.java
index 65bebc6c235af..1a47aa5cd7773 100644
--- a/services/core/java/com/android/server/am/UserController.java
+++ b/services/core/java/com/android/server/am/UserController.java
@@ -795,11 +795,13 @@ class UserController implements Handler.Callback {
      */
     private void stopGuestOrEphemeralUserIfBackground(int oldUserId) {
         if (DEBUG_MU) Slog.i(TAG, "Stop guest or ephemeral user if background: " + oldUserId);
-        UserState oldUss = mStartedUsers.get(oldUserId);
-        if (oldUserId == UserHandle.USER_SYSTEM || oldUserId == mCurrentUserId || oldUss == null
-                || oldUss.state == UserState.STATE_STOPPING
-                || oldUss.state == UserState.STATE_SHUTDOWN) {
-            return;
+        synchronized(mLock) {
+            UserState oldUss = mStartedUsers.get(oldUserId);
+            if (oldUserId == UserHandle.USER_SYSTEM || oldUserId == mCurrentUserId || oldUss == null
+                    || oldUss.state == UserState.STATE_STOPPING
+                    || oldUss.state == UserState.STATE_SHUTDOWN) {
+                return;
+            }
         }
 
         UserInfo userInfo = getUserInfo(oldUserId);