diff --git a/lib/browser/mqtt5_utils.spec.ts b/lib/browser/mqtt5_utils.spec.ts
index 56d5c6d13..d0545da69 100644
--- a/lib/browser/mqtt5_utils.spec.ts
+++ b/lib/browser/mqtt5_utils.spec.ts
@@ -353,7 +353,6 @@ function create_base_expected_mqtt_js_config() : mqtt.IClientOptions {
         autoAssignTopicAlias : false,
         transformWsUrl: undefined, /* TOFIX */
         resubscribe : false,
-        clientId : ""
     };
 }
 
diff --git a/lib/browser/mqtt5_utils.ts b/lib/browser/mqtt5_utils.ts
index 6470acab7..ebc8151f5 100644
--- a/lib/browser/mqtt5_utils.ts
+++ b/lib/browser/mqtt5_utils.ts
@@ -226,10 +226,10 @@ export function create_mqtt_js_client_config_from_crt_client_config(crtConfig :
     }
 
     /*
-     * If you leave clientId undefined, mqtt-js will make up some weird thing for you, but the intent is that it
-     * should pass the empty client id so that the server assigns you one.
+     * MQTT.js treats an undefined client id specially, and we want it to do so, otherwise it will refuse
+     * to connect with clean session false
      */
-    utils.set_defined_property(mqttJsClientConfig, "clientId", crtConfig.connectProperties?.clientId ?? "");
+    utils.set_defined_property(mqttJsClientConfig, "clientId", crtConfig.connectProperties?.clientId);
     utils.set_defined_property(mqttJsClientConfig, "username", crtConfig.connectProperties?.username);
     utils.set_defined_property(mqttJsClientConfig, "password", crtConfig.connectProperties?.password);
     utils.set_defined_property(mqttJsClientConfig, "will", create_mqtt_js_will_from_crt_config(crtConfig.connectProperties));