diff --git a/openbsc/include/openbsc/gprs_gsup_messages.h b/openbsc/include/openbsc/gprs_gsup_messages.h
index b63f74b..9857b97 100644
--- a/openbsc/include/openbsc/gprs_gsup_messages.h
+++ b/openbsc/include/openbsc/gprs_gsup_messages.h
@@ -74,6 +74,10 @@
 	GPRS_GSUP_MSGT_LOCATION_CANCEL_RESULT	= 0b00011110,
 };
 
+#define GPRS_GSUP_IS_MSGT_REQUEST(msgt) (((msgt) & 0b00000011) == 0b00)
+#define GPRS_GSUP_IS_MSGT_ERROR(msgt)   (((msgt) & 0b00000011) == 0b01)
+#define GPRS_GSUP_TO_MSGT_ERROR(msgt)   (((msgt) & 0b11111100) | 0b01)
+
 enum gprs_gsup_cancel_type {
 	GPRS_GSUP_CANCEL_TYPE_UPDATE		= 1, /* on wire: 0 */
 	GPRS_GSUP_CANCEL_TYPE_WITHDRAW		= 2, /* on wire: 1 */
diff --git a/openbsc/include/openbsc/gprs_sgsn.h b/openbsc/include/openbsc/gprs_sgsn.h
index d6a9bda..25810ab 100644
--- a/openbsc/include/openbsc/gprs_sgsn.h
+++ b/openbsc/include/openbsc/gprs_sgsn.h
@@ -289,7 +289,8 @@
 };
 
 #define LOGGSUBSCRP(level, subscr, fmt, args...) \
-	LOGP(DGPRS, level, "SUBSCR(%s) " fmt, (subscr)->imsi, \
+	LOGP(DGPRS, level, "SUBSCR(%s) " fmt, \
+	     (subscr) ? (subscr)->imsi : "---", \
 	     ## args)
 
 struct sgsn_config;
diff --git a/openbsc/src/gprs/gprs_subscriber.c b/openbsc/src/gprs/gprs_subscriber.c
index 88e037e..e971210 100644
--- a/openbsc/src/gprs/gprs_subscriber.c
+++ b/openbsc/src/gprs/gprs_subscriber.c
@@ -231,12 +231,13 @@
 {
 	struct msgb *msg = gprs_gsup_msgb_alloc();
 
-	strncpy(gsup_msg->imsi, subscr->imsi, sizeof(gsup_msg->imsi) - 1);
+	if (strlen(gsup_msg->imsi) == 0 && subscr)
+		strncpy(gsup_msg->imsi, subscr->imsi, sizeof(gsup_msg->imsi) - 1);
 
 	gprs_gsup_encode(msg, gsup_msg);
 
 	LOGGSUBSCRP(LOGL_INFO, subscr,
-		"Sending GSUP, will send: %s\n", msgb_hexdump(msg));
+		    "Sending GSUP, will send: %s\n", msgb_hexdump(msg));
 
 	if (!sgsn->gsup_client) {
 		msgb_free(msg);
@@ -246,6 +247,20 @@
 	return gprs_gsup_client_send(sgsn->gsup_client, msg);
 }
 
+static int gprs_subscr_tx_gsup_error_reply(struct gsm_subscriber *subscr,
+					   struct gprs_gsup_message *gsup_orig,
+					   enum gsm48_gmm_cause cause)
+{
+	struct gprs_gsup_message gsup_reply = {0};
+
+	strncpy(gsup_reply.imsi, gsup_orig->imsi, sizeof(gsup_reply.imsi) - 1);
+	gsup_reply.cause = cause;
+	gsup_reply.message_type =
+		GPRS_GSUP_TO_MSGT_ERROR(gsup_orig->message_type);
+
+	return gprs_subscr_tx_gsup_message(subscr, &gsup_reply);
+}
+
 static int gprs_subscr_handle_gsup_auth_res(struct gsm_subscriber *subscr,
 					    struct gprs_gsup_message *gsup_msg)
 {
@@ -475,6 +490,32 @@
 	return -gsup_msg->cause;
 }
 
+static int gprs_subscr_handle_unknown_imsi(struct gprs_gsup_message *gsup_msg)
+{
+	if (GPRS_GSUP_IS_MSGT_REQUEST(gsup_msg->message_type)) {
+		gprs_subscr_tx_gsup_error_reply(NULL, gsup_msg,
+						GMM_CAUSE_IMSI_UNKNOWN);
+		LOGP(DGPRS, LOGL_NOTICE,
+		     "Unknown IMSI %s, discarding GSUP request "
+		     "of type 0x%02x\n",
+		     gsup_msg->imsi, gsup_msg->message_type);
+	} else if (GPRS_GSUP_IS_MSGT_ERROR(gsup_msg->message_type)) {
+		LOGP(DGPRS, LOGL_NOTICE,
+		     "Unknown IMSI %s, discarding GSUP error "
+		     "of type 0x%02x, cause '%s' (%d)\n",
+		     gsup_msg->imsi, gsup_msg->message_type,
+		     get_value_string(gsm48_gmm_cause_names, gsup_msg->cause),
+		     gsup_msg->cause);
+	} else {
+		LOGP(DGPRS, LOGL_NOTICE,
+		     "Unknown IMSI %s, discarding GSUP response "
+		     "of type 0x%02x\n",
+		     gsup_msg->imsi, gsup_msg->message_type);
+	}
+
+	return -GMM_CAUSE_IMSI_UNKNOWN;
+}
+
 int gprs_subscr_rx_gsup_message(struct msgb *msg)
 {
 	uint8_t *data = msgb_l2(msg);
@@ -500,11 +541,8 @@
 	else
 		subscr = gprs_subscr_get_by_imsi(gsup_msg.imsi);
 
-	if (!subscr) {
-		LOGP(DGPRS, LOGL_NOTICE,
-		     "Unknown IMSI %s, discarding GSUP message\n", gsup_msg.imsi);
-		return -GMM_CAUSE_IMSI_UNKNOWN;
-	}
+	if (!subscr)
+		return gprs_subscr_handle_unknown_imsi(&gsup_msg);
 
 	LOGGSUBSCRP(LOGL_INFO, subscr,
 		"Received GSUP message of type 0x%02x\n", gsup_msg.message_type);
@@ -545,6 +583,8 @@
 		LOGGSUBSCRP(LOGL_ERROR, subscr,
 			"Rx GSUP message type %d not yet implemented\n",
 			gsup_msg.message_type);
+		gprs_subscr_tx_gsup_error_reply(subscr, &gsup_msg,
+						GMM_CAUSE_MSGT_NOTEXIST_NOTIMPL);
 		rc = -GMM_CAUSE_MSGT_NOTEXIST_NOTIMPL;
 		break;
 
@@ -552,7 +592,10 @@
 		LOGGSUBSCRP(LOGL_ERROR, subscr,
 			"Rx GSUP message type %d not valid at SGSN\n",
 			gsup_msg.message_type);
-		rc = -GMM_CAUSE_MSGT_INCOMP_P_STATE;
+		if (GPRS_GSUP_IS_MSGT_REQUEST(gsup_msg.message_type))
+			gprs_subscr_tx_gsup_error_reply(
+				subscr, &gsup_msg, GMM_CAUSE_MSGT_NOTEXIST_NOTIMPL);
+		rc = -GMM_CAUSE_MSGT_NOTEXIST_NOTIMPL;
 		break;
 	};
 
diff --git a/openbsc/tests/sgsn/sgsn_test.c b/openbsc/tests/sgsn/sgsn_test.c
index d419f0a..7050a16 100644
--- a/openbsc/tests/sgsn/sgsn_test.c
+++ b/openbsc/tests/sgsn/sgsn_test.c
@@ -468,6 +468,21 @@
 		0x06, 0x01, 0x00,
 	};
 
+	static const uint8_t insert_data_req[] = {
+		0x10,
+		TEST_GSUP_IMSI1_IE,
+		0x05, 0x11,
+			0x10, 0x01, 0x03,
+			0x11, 0x02, 0xf1, 0x21, /* IPv4 */
+			0x12, 0x08, 0x03, 'b', 'a', 'r', 0x03, 'a', 'p', 'n',
+	};
+
+	static const uint8_t delete_data_req[] = {
+		0x14,
+		TEST_GSUP_IMSI1_IE,
+		0x10, 0x01, 0x03,
+	};
+
 	printf("Testing subcriber GSUP handling\n");
 
 	update_subscriber_data_cb = my_dummy_sgsn_update_subscriber_data;
@@ -527,7 +542,19 @@
 	/* Check authorization */
 	OSMO_ASSERT(s1->authorized == 0);
 
-	/* Inject UpdateLocReq GSUP message */
+	/* Inject InsertSubscrData GSUP message */
+	last_updated_subscr = NULL;
+	rc = rx_gsup_message(insert_data_req, sizeof(insert_data_req));
+	OSMO_ASSERT(rc == -GMM_CAUSE_MSGT_NOTEXIST_NOTIMPL);
+	OSMO_ASSERT(last_updated_subscr == NULL);
+
+	/* Inject DeleteSubscrData GSUP message */
+	last_updated_subscr = NULL;
+	rc = rx_gsup_message(delete_data_req, sizeof(delete_data_req));
+	OSMO_ASSERT(rc == -GMM_CAUSE_MSGT_NOTEXIST_NOTIMPL);
+	OSMO_ASSERT(last_updated_subscr == NULL);
+
+	/* Inject LocCancelReq GSUP message */
 	rc = rx_gsup_message(location_cancellation_req,
 			     sizeof(location_cancellation_req));
 	OSMO_ASSERT(rc >= 0);
@@ -543,6 +570,24 @@
 	OSMO_ASSERT(s1found == NULL);
 	gprs_llgmm_assign(llme, local_tlli, 0xffffffff, GPRS_ALGO_GEA0, NULL);
 
+	/* Inject InsertSubscrData GSUP message (unknown IMSI) */
+	last_updated_subscr = NULL;
+	rc = rx_gsup_message(insert_data_req, sizeof(insert_data_req));
+	/* TODO: Remove the comments when this is fixed */
+	/* OSMO_ASSERT(rc == -GMM_CAUSE_IMSI_UNKNOWN); */
+	OSMO_ASSERT(last_updated_subscr == NULL);
+
+	/* Inject DeleteSubscrData GSUP message (unknown IMSI) */
+	rc = rx_gsup_message(delete_data_req, sizeof(delete_data_req));
+	OSMO_ASSERT(rc == -GMM_CAUSE_IMSI_UNKNOWN);
+	OSMO_ASSERT(last_updated_subscr == NULL);
+
+	/* Inject LocCancelReq GSUP message (unknown IMSI) */
+	rc = rx_gsup_message(location_cancellation_req,
+			     sizeof(location_cancellation_req));
+	OSMO_ASSERT(rc == -GMM_CAUSE_IMSI_UNKNOWN);
+	OSMO_ASSERT(last_updated_subscr == NULL);
+
 	update_subscriber_data_cb = __real_sgsn_update_subscriber_data;
 }