diff --git a/Tethering/src/com/android/networkstack/tethering/Tethering.java b/Tethering/src/com/android/networkstack/tethering/Tethering.java
index b3ec805..e1b7016 100644
--- a/Tethering/src/com/android/networkstack/tethering/Tethering.java
+++ b/Tethering/src/com/android/networkstack/tethering/Tethering.java
@@ -2407,6 +2407,9 @@
 
     /** Unregister tethering event callback */
     void unregisterTetheringEventCallback(ITetheringEventCallback callback) {
+        if (callback == null) {
+            throw new NullPointerException();
+        }
         mHandler.post(() -> {
             mTetheringEventCallbacks.unregister(callback);
         });