diff --git a/binder_service_android.h b/binder_service_android.h
index 3293c0e..375dad4 100644
--- a/binder_service_android.h
+++ b/binder_service_android.h
@@ -61,6 +61,9 @@
   android::binder::Status bind(
       const android::sp<android::os::IUpdateEngineCallback>& callback,
       bool* return_value) override;
+  android::binder::Status unbind(
+      const android::sp<android::os::IUpdateEngineCallback>& callback,
+      bool* return_value) override;
   android::binder::Status suspend() override;
   android::binder::Status resume() override;
   android::binder::Status cancel() override;
@@ -68,8 +71,9 @@
 
  private:
   // Remove the passed |callback| from the list of registered callbacks. Called
-  // whenever the callback object is destroyed.
-  void UnbindCallback(android::os::IUpdateEngineCallback* callback);
+  // on unbind() or whenever the callback object is destroyed.
+  // Returns true on success.
+  bool UnbindCallback(android::os::IUpdateEngineCallback* callback);
 
   // List of currently bound callbacks.
   std::vector<android::sp<android::os::IUpdateEngineCallback>> callbacks_;