diff --git a/automotive/vehicle/2.0/types.hal b/automotive/vehicle/2.0/types.hal
index 7a5f2d2..bce42b9 100644
--- a/automotive/vehicle/2.0/types.hal
+++ b/automotive/vehicle/2.0/types.hal
@@ -4114,7 +4114,8 @@
 /**
  * Id of an Android user.
  *
- * Must be > 0 for valid ids, or -1 when it's not used.
+ * Must be > 0 for valid ids, or -10000 (which is the same as Android.UserHandle.USER_NULL) when
+ * it's not used.
  */
 typedef int32_t UserId;