--- xfce4-settings-4.12.0.orig/dialogs/display-settings/xfce-randr.c
+++ xfce4-settings-4.12.0/dialogs/display-settings/xfce-randr.c
@@ -628,7 +628,7 @@ xfce_randr_preferred_mode (XfceRandr *ra
     {
         if (n < randr->priv->output_info[output]->npreferred)
             dist = 0;
-        else if (randr->priv->output_info[output]->mm_height != 0)
+        else if (randr->priv->output_info[output]->mm_height != 0 && gdk_screen_height_mm () != 0)
             dist = (1000 * gdk_screen_height () / gdk_screen_height_mm () -
                 1000 * randr->priv->modes[output][n].height /
                     randr->priv->output_info[output]->mm_height);