diff --git a/openbsc/src/gprs/gprs_gmm.c b/openbsc/src/gprs/gprs_gmm.c
index dac6c1b..48d162f 100644
--- a/openbsc/src/gprs/gprs_gmm.c
+++ b/openbsc/src/gprs/gprs_gmm.c
@@ -1245,7 +1245,8 @@
 
 	if (!mmctx) {
 		LOGP(DMM, LOGL_NOTICE, "Cannot handle SM for unknown MM CTX\n");
-		return gsm48_tx_gmm_status_oldmsg(msg, GSM_CAUSE_PROTO_ERR_UNSPEC);
+		gsm48_tx_gmm_status_oldmsg(msg, GMM_CAUSE_IMPL_DETACHED);
+		return gsm48_tx_sm_status_oldmsg(msg, GSM_CAUSE_PROTO_ERR_UNSPEC);
 	}
 
 	switch (gh->msg_type) {