diff --git a/gui-lib/mred/private/wx/gtk/resolution.rkt b/gui-lib/mred/private/wx/gtk/resolution.rkt
index c387b572..ab22f615 100644
--- a/gui-lib/mred/private/wx/gtk/resolution.rkt
+++ b/gui-lib/mred/private/wx/gtk/resolution.rkt
@@ -43,4 +43,6 @@
 	 (g_variant_get_double
 	  (g_settings_get_value gs "text-scaling-factor"))))
     (g_object_unref gs)
-    v))
+    (and (rational? v)
+	 (positive? v)
+	 v)))