diff --git a/gtk/gtktextmark.h b/gtk/gtktextmark.h
index 7eb80f0599..87118df7ec 100644
--- a/gtk/gtktextmark.h
+++ b/gtk/gtktextmark.h
@@ -72,7 +72,7 @@ struct _GtkTextMark
 {
   GObject parent_instance;
 
-  gpointer segment;
+  gpointer GSEAL (segment);
 };
 
 struct _GtkTextMarkClass