diff --git a/include/android/storage_manager.h b/include/android/storage_manager.h
index 7f2ee08..270570e 100644
--- a/include/android/storage_manager.h
+++ b/include/android/storage_manager.h
@@ -124,6 +124,12 @@
 
 /**
  * Attempts to mount an OBB file. This is an asynchronous operation.
+ *
+ * Since API level 33, this function can only be used to mount unencrypted OBBs,
+ * i.e. the {@code key} parameter must be {@code null} or an empty string. Note
+ * that even before API level 33, mounting encrypted OBBs didn't work on many
+ * Android device implementations. Applications should not assume any particular
+ * behavior when {@code key} is nonempty.
  */
 void AStorageManager_mountObb(AStorageManager* mgr, const char* filename, const char* key,
         AStorageManager_obbCallbackFunc cb, void* data);