diff --git a/openbsc/src/libmsc/smpp_openbsc.c b/openbsc/src/libmsc/smpp_openbsc.c
index 1671a62..ec9dda3 100644
--- a/openbsc/src/libmsc/smpp_openbsc.c
+++ b/openbsc/src/libmsc/smpp_openbsc.c
@@ -559,7 +559,7 @@
 
 	esme = smpp_route(g_smsc, &dst);
 	if (!esme)
-		return 1; /* unknown subscriber */
+		return GSM411_RP_CAUSE_MO_NUM_UNASSIGNED;
 
 	return deliver_to_esme(esme, sms, conn);
 }