diff --git a/dbms/src/Storages/DeltaMerge/BitmapFilter/BitmapFilterBlockInputStream.h b/dbms/src/Storages/DeltaMerge/BitmapFilter/BitmapFilterBlockInputStream.h
index 95b0f874d17..fc8125e70b1 100644
--- a/dbms/src/Storages/DeltaMerge/BitmapFilter/BitmapFilterBlockInputStream.h
+++ b/dbms/src/Storages/DeltaMerge/BitmapFilter/BitmapFilterBlockInputStream.h
@@ -44,7 +44,9 @@ class BitmapFilterBlockInputStream : public IProfilingBlockInputStream
         FilterPtr filter_ignored;
         return readImpl(filter_ignored, false);
     }
-
+    // When all rows in block are not filtered out,
+    // `res_filter` will be set to null.
+    // The caller needs to do handle this situation.
     Block readImpl(FilterPtr & res_filter, bool return_filter) override;
 
 private: