diff --git a/libnm/nm-object.c b/libnm/nm-object.c
index cab92e9b9..e97a14ab1 100644
--- a/libnm/nm-object.c
+++ b/libnm/nm-object.c
@@ -498,9 +498,12 @@ _nm_object_class_add_interface (NMObjectClass *object_class,
 	NMObjectClassPrivate *cpriv;
 
 	g_return_if_fail (NM_IS_OBJECT_CLASS (object_class));
+	g_return_if_fail (interface);
 
 	cpriv = NM_OBJECT_CLASS_GET_PRIVATE (object_class);
 
+	g_return_if_fail (g_slist_find_custom (cpriv->interfaces, interface, (GCompareFunc) g_strcmp0) == NULL);
+
 	cpriv->interfaces = g_slist_prepend (cpriv->interfaces, g_strdup (interface));
 }