diff --git a/lib/connect.c b/lib/connect.c
index 6fbc776e..0de00322 100644
--- a/lib/connect.c
+++ b/lib/connect.c
@@ -33,9 +33,27 @@ static int mosquitto__connect_init(struct mosquitto *mosq, const char *host, int
 
 static int mosquitto__connect_init(struct mosquitto *mosq, const char *host, int port, int keepalive, const char *bind_address)
 {
+	int i;
+
 	if(!mosq) return MOSQ_ERR_INVAL;
 	if(!host || port <= 0) return MOSQ_ERR_INVAL;
 
+	if(mosq->id == NULL && (mosq->protocol == mosq_p_mqtt31 || mosq->protocol == mosq_p_mqtt311)){
+		mosq->id = (char *)mosquitto__calloc(24, sizeof(char));
+		if(!mosq->id){
+			return MOSQ_ERR_NOMEM;
+		}
+		mosq->id[0] = 'm';
+		mosq->id[1] = 'o';
+		mosq->id[2] = 's';
+		mosq->id[3] = 'q';
+		mosq->id[4] = '/';
+
+		for(i=5; i<23; i++){
+			mosq->id[i] = (random()%73)+48;
+		}
+	}
+
 	mosquitto__free(mosq->host);
 	mosq->host = mosquitto__strdup(host);
 	if(!mosq->host) return MOSQ_ERR_NOMEM;
diff --git a/lib/mosquitto.c b/lib/mosquitto.c
index 688312d3..92c4e9ea 100644
--- a/lib/mosquitto.c
+++ b/lib/mosquitto.c
@@ -104,8 +104,6 @@ struct mosquitto *mosquitto_new(const char *id, bool clean_start, void *userdata
 
 int mosquitto_reinitialise(struct mosquitto *mosq, const char *id, bool clean_start, void *userdata)
 {
-	int i;
-
 	if(!mosq) return MOSQ_ERR_INVAL;
 
 	if(clean_start == false && id == NULL){
@@ -134,20 +132,6 @@ int mosquitto_reinitialise(struct mosquitto *mosq, const char *id, bool clean_st
 			return MOSQ_ERR_MALFORMED_UTF8;
 		}
 		mosq->id = mosquitto__strdup(id);
-	}else{
-		mosq->id = (char *)mosquitto__calloc(24, sizeof(char));
-		if(!mosq->id){
-			return MOSQ_ERR_NOMEM;
-		}
-		mosq->id[0] = 'm';
-		mosq->id[1] = 'o';
-		mosq->id[2] = 's';
-		mosq->id[3] = 'q';
-		mosq->id[4] = '/';
-
-		for(i=5; i<23; i++){
-			mosq->id[i] = (rand()%73)+48;
-		}
 	}
 	mosq->in_packet.payload = NULL;
 	packet__cleanup(&mosq->in_packet);
diff --git a/lib/send_connect.c b/lib/send_connect.c
index a052a398..5b50e2c7 100644
--- a/lib/send_connect.c
+++ b/lib/send_connect.c
@@ -44,7 +44,8 @@ int send__connect(struct mosquitto *mosq, uint16_t keepalive, bool clean_session
 	int proplen, varbytes;
 
 	assert(mosq);
-	assert(mosq->id);
+
+	if(mosq->protocol == mosq_p_mqtt31 && !mosq->id) return MOSQ_ERR_PROTOCOL;
 
 #if defined(WITH_BROKER) && defined(WITH_BRIDGE)
 	if(mosq->bridge){
@@ -81,7 +82,11 @@ int send__connect(struct mosquitto *mosq, uint16_t keepalive, bool clean_session
 	packet = mosquitto__calloc(1, sizeof(struct mosquitto__packet));
 	if(!packet) return MOSQ_ERR_NOMEM;
 
-	payloadlen = 2+strlen(clientid);
+	if(clientid){
+		payloadlen = 2+strlen(clientid);
+	}else{
+		payloadlen = 2;
+	}
 	if(mosq->will){
 		will = 1;
 		assert(mosq->will->msg.topic);
@@ -140,7 +145,11 @@ int send__connect(struct mosquitto *mosq, uint16_t keepalive, bool clean_session
 	}
 
 	/* Payload */
-	packet__write_string(packet, clientid, strlen(clientid));
+	if(clientid){
+		packet__write_string(packet, clientid, strlen(clientid));
+	}else{
+		packet__write_uint16(packet, 0);
+	}
 	if(will){
 		if(mosq->protocol == mosq_p_mqtt5){
 			/* Write will properties */
diff --git a/test/broker/01-connect-invalid-id-missing.py b/test/broker/01-connect-invalid-id-missing.py
index 32188a12..63273e93 100755
--- a/test/broker/01-connect-invalid-id-missing.py
+++ b/test/broker/01-connect-invalid-id-missing.py
@@ -6,13 +6,14 @@ from mosq_test_helper import *
 
 rc = 1
 keepalive = 10
-connect_packet = mosq_test.gen_connect(None, keepalive=keepalive)
+connect_packet = mosq_test.gen_connect(None, keepalive=keepalive, proto_ver=3)
+connack_packet = mosq_test.gen_connack(rc=2)
 
 port = mosq_test.get_port()
 broker = mosq_test.start_broker(filename=os.path.basename(__file__), port=port)
 
 try:
-    sock = mosq_test.do_client_connect(connect_packet, "", port=port)
+    sock = mosq_test.do_client_connect(connect_packet, connack_packet, port=port)
     sock.close()
     rc = 0
 finally:
diff --git a/test/broker/12-prop-assigned-client-identifier.py b/test/broker/12-prop-assigned-client-identifier.py
new file mode 100755
index 00000000..fac50d11
--- /dev/null
+++ b/test/broker/12-prop-assigned-client-identifier.py
@@ -0,0 +1,48 @@
+#!/usr/bin/env python
+
+# Test whether sending a non zero session expiry interval in DISCONNECT after
+# having sent a zero session expiry interval is treated correctly in MQTT v5.
+
+from mosq_test_helper import *
+
+rc = 1
+
+keepalive = 10
+
+
+connect_packet = mosq_test.gen_connect(None, proto_ver=5, keepalive=keepalive)
+
+props = mqtt5_props.gen_string_prop(mqtt5_props.PROP_ASSIGNED_CLIENT_IDENTIFIER, "auto-00000000-0000-0000-0000-000000000000")
+connack_packet = mosq_test.gen_connack(rc=0, proto_ver=5, properties=props)
+
+props = mqtt5_props.gen_uint32_prop(mqtt5_props.PROP_SESSION_EXPIRY_INTERVAL, 1)
+props = mqtt5_props.prop_finalise(props)
+disconnect_client_packet = mosq_test.gen_disconnect(proto_ver=5, properties=props)
+
+disconnect_server_packet = mosq_test.gen_disconnect(proto_ver=5, reason_code=130)
+
+port = mosq_test.get_port()
+broker = mosq_test.start_broker(filename=os.path.basename(__file__), port=port)
+
+try:
+    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+    sock.settimeout(10)
+    sock.connect(("localhost", port))
+
+    sock.send(connect_packet)
+    connack_recvd = sock.recv(len(connack_packet))
+
+    if connack_recvd[0:12] == connack_packet[0:12]:
+        # FIXME - this test could be tightened up a lot
+        rc = 0
+    
+    sock.close()
+finally:
+    broker.terminate()
+    broker.wait()
+    (stdo, stde) = broker.communicate()
+    if rc:
+        print(stde)
+
+exit(rc)
+
diff --git a/test/broker/Makefile b/test/broker/Makefile
index f19566ea..db0b971a 100644
--- a/test/broker/Makefile
+++ b/test/broker/Makefile
@@ -139,3 +139,4 @@ endif
 	./12-prop-session-expiry-invalid.py
 	./12-prop-subpub-payload-format.py
 	./12-prop-subpub-content-type.py
+	./12-prop-assigned-client-identifier.py
diff --git a/test/broker/ptest.py b/test/broker/ptest.py
index 5d9fb0a3..ff2984fd 100755
--- a/test/broker/ptest.py
+++ b/test/broker/ptest.py
@@ -108,6 +108,7 @@ tests = [
 	(1, './12-prop-session-expiry-invalid.py'),
 	(1, './12-prop-subpub-payload-format.py'),
 	(1, './12-prop-subpub-content-type.py'),
+	(1, './12-prop-assigned-client-identifier.py'),
     ]
 
 minport = 1888
diff --git a/test/mosq_test.py b/test/mosq_test.py
index cc3e9264..8d29d1a5 100644
--- a/test/mosq_test.py
+++ b/test/mosq_test.py
@@ -308,6 +308,8 @@ def gen_connect(client_id, clean_session=True, keepalive=60, username=None, pass
 
     if client_id != None:
         remaining_length = remaining_length + 2+len(client_id)
+    else:
+        remaining_length = remaining_length + 2
 
     connect_flags = 0
 
@@ -347,6 +349,8 @@ def gen_connect(client_id, clean_session=True, keepalive=60, username=None, pass
 
     if client_id != None:
         packet = packet + struct.pack("!H"+str(len(client_id))+"s", len(client_id), client_id)
+    else:
+        packet = packet + struct.pack("!H", 0)
 
     if will_topic != None:
         packet = packet + struct.pack("!H"+str(len(will_topic))+"s", len(will_topic), will_topic)