diff --git a/gdk/x11/gdkwindow-x11.c b/gdk/x11/gdkwindow-x11.c
index b2e314d58a..5744d66029 100644
--- a/gdk/x11/gdkwindow-x11.c
+++ b/gdk/x11/gdkwindow-x11.c
@@ -3587,8 +3587,8 @@ gdk_x11_window_set_frame_extents (GdkWindow *window,
  * Since: 3.2
  */
 void
-gdk_x11_window_set_theme_variant (GdkWindow *window,
-                                  char      *variant)
+gdk_x11_window_set_theme_variant (GdkWindow  *window,
+                                  const char *variant)
 {
   gdk_x11_window_set_utf8_property (window, "_GTK_THEME_VARIANT",
                                     variant ? variant : "");
diff --git a/gdk/x11/gdkx11window.h b/gdk/x11/gdkx11window.h
index d366d3b457..b4fac2ff90 100644
--- a/gdk/x11/gdkx11window.h
+++ b/gdk/x11/gdkx11window.h
@@ -64,7 +64,7 @@ void     gdk_x11_window_set_utf8_property    (GdkWindow *window,
 					      const gchar *value);
 GDK_AVAILABLE_IN_3_2
 void     gdk_x11_window_set_theme_variant (GdkWindow   *window,
-                                           char        *variant);
+                                           const char  *variant);
 GDK_DEPRECATED_IN_3_12_FOR(gdk_window_set_shadow_width)
 void     gdk_x11_window_set_frame_extents (GdkWindow *window,
                                            int        left,