diff --git a/lib/sp-visualizer-row.h b/lib/sp-visualizer-row.h
index 137fdd8b..fb01565c 100644
--- a/lib/sp-visualizer-row.h
+++ b/lib/sp-visualizer-row.h
@@ -42,6 +42,23 @@ struct _SpVisualizerRowClass
    */
   void (*set_reader) (SpVisualizerRow *self,
                       SpCaptureReader *reader);
+
+  gpointer _reserved1;
+  gpointer _reserved2;
+  gpointer _reserved3;
+  gpointer _reserved4;
+  gpointer _reserved5;
+  gpointer _reserved6;
+  gpointer _reserved7;
+  gpointer _reserved8;
+  gpointer _reserved9;
+  gpointer _reserved10;
+  gpointer _reserved11;
+  gpointer _reserved12;
+  gpointer _reserved13;
+  gpointer _reserved14;
+  gpointer _reserved15;
+  gpointer _reserved16;
 };
 
 void sp_visualizer_row_set_reader (SpVisualizerRow *self,