diff --git a/include/sampleflow/filter.h b/include/sampleflow/filter.h
index f8734af9..87c3568d 100644
--- a/include/sampleflow/filter.h
+++ b/include/sampleflow/filter.h
@@ -131,7 +131,7 @@ namespace SampleFlow
        */
       virtual
       void
-      flush ();
+      flush () override;
 
       /**
        * The main function of this class, which needs to be implemented by