diff --git a/gthumb/gth-string-list.c b/gthumb/gth-string-list.c
index e159403..42fa8e6 100644
--- a/gthumb/gth-string-list.c
+++ b/gthumb/gth-string-list.c
@@ -190,6 +190,9 @@ gth_string_list_append (GthStringList *list1,
 {
 	GList *scan;
 
+	if (list1 == NULL || list2 == NULL)
+		return;
+
 	for (scan = list2->priv->list; scan; scan = scan->next)
 		if (! g_list_find_custom (list1->priv->list, scan->data, (GCompareFunc) strcmp))
 			list1->priv->list = g_list_append (list1->priv->list, g_strdup (scan->data));