diff --git a/gtk/gtkcolordialogbutton.c b/gtk/gtkcolordialogbutton.c
index 6e0db26f01..139d9f7eb1 100644
--- a/gtk/gtkcolordialogbutton.c
+++ b/gtk/gtkcolordialogbutton.c
@@ -401,7 +401,7 @@ gtk_color_dialog_button_new (GtkColorDialog *dialog)
 {
   GtkWidget *self;
 
-  g_return_val_if_fail (GTK_IS_COLOR_DIALOG (dialog), NULL);
+  g_return_val_if_fail (dialog == NULL || GTK_IS_COLOR_DIALOG (dialog), NULL);
 
   self = g_object_new (GTK_TYPE_COLOR_DIALOG_BUTTON,
                        "dialog", dialog,
diff --git a/gtk/gtkfontdialogbutton.c b/gtk/gtkfontdialogbutton.c
index ff466a2003..ab6c78ac92 100644
--- a/gtk/gtkfontdialogbutton.c
+++ b/gtk/gtkfontdialogbutton.c
@@ -697,7 +697,7 @@ gtk_font_dialog_button_new (GtkFontDialog *dialog)
 {
   GtkWidget *self;
 
-  g_return_val_if_fail (GTK_IS_FONT_DIALOG (dialog), NULL);
+  g_return_val_if_fail (dialog == NULL || GTK_IS_FONT_DIALOG (dialog), NULL);
 
   self = g_object_new (GTK_TYPE_FONT_DIALOG_BUTTON,
                        "dialog", dialog,