diff --git a/src/devices/nm-device.c b/src/devices/nm-device.c
index b8b4823f9f..afc81dcde7 100644
--- a/src/devices/nm-device.c
+++ b/src/devices/nm-device.c
@@ -5278,9 +5278,14 @@ tc_commit (NMDevice *self)
 			if (action) {
 				tfilter->action.kind = nm_tc_action_get_kind (action);
 				if (strcmp (tfilter->action.kind, "simple") == 0) {
-					strncpy (tfilter->action.simple.sdata,
-					         g_variant_get_bytestring (nm_tc_action_get_attribute (action, "sdata")),
-					         sizeof (tfilter->action.simple.sdata));
+					GVariant *sdata;
+
+					sdata = nm_tc_action_get_attribute (action, "sdata");
+					if (sdata && g_variant_is_of_type (sdata, G_VARIANT_TYPE_BYTESTRING)) {
+						g_strlcpy (tfilter->action.simple.sdata,
+						           g_variant_get_bytestring (sdata),
+						           sizeof (tfilter->action.simple.sdata));
+					}
 				}
 			}