diff --git a/gtk/gtktextview.c b/gtk/gtktextview.c
index a869fb0ba2..55101c6069 100644
--- a/gtk/gtktextview.c
+++ b/gtk/gtktextview.c
@@ -5068,7 +5068,7 @@ move_cursor (GtkTextView       *text_view,
 }
 
 static gboolean
-iter_line_is_rtl (GtkTextIter *iter)
+iter_line_is_rtl (const GtkTextIter *iter)
 {
   GtkTextIter start, end;
   char *text;