diff --git a/src/libmsc/gsm_04_11_gsup.c b/src/libmsc/gsm_04_11_gsup.c
index a1ec634..b4d74c7 100644
--- a/src/libmsc/gsm_04_11_gsup.c
+++ b/src/libmsc/gsm_04_11_gsup.c
@@ -300,6 +300,8 @@
 	struct vlr_subscr *vsub = vlr_subscr_find_by_imsi(vlr, gsup_msg->imsi, __func__);
 
 	if (!vsub) {
+		LOGP(DLSMS, LOGL_ERROR, "Rx %s for unknown subscriber, rejecting\n",
+		     osmo_gsup_message_type_name(gsup_msg->message_type));
 		gsup_client_mux_tx_error_reply(gcm, gsup_msg, GMM_CAUSE_IMSI_UNKNOWN);
 		return -GMM_CAUSE_IMSI_UNKNOWN;
 	}
diff --git a/src/libmsc/gsm_09_11.c b/src/libmsc/gsm_09_11.c
index 6558272..3de933c 100644
--- a/src/libmsc/gsm_09_11.c
+++ b/src/libmsc/gsm_09_11.c
@@ -443,6 +443,8 @@
 	struct vlr_subscr *vsub = vlr_subscr_find_by_imsi(vlr, gsup_msg->imsi, __func__);
 
 	if (!vsub) {
+		LOGP(DSS, LOGL_ERROR, "Rx %s for unknown subscriber, rejecting\n",
+		     osmo_gsup_message_type_name(gsup_msg->message_type));
 		gsup_client_mux_tx_error_reply(gcm, gsup_msg, GMM_CAUSE_IMSI_UNKNOWN);
 		return -GMM_CAUSE_IMSI_UNKNOWN;
 	}