improve error message

lib/standalone.js

@@ -27,7 +27,7 @@ function LEX(obj, app) {
   }
 
   obj.debug = LEX.debug;
-  
+
   if ('function' === typeof app) {
     obj.onRequest = obj.onRequest || app;
   }
@@ -162,6 +162,10 @@ function LEX(obj, app) {
   }
 
   function listen(plainPorts, tlsPorts, onListening) {
+    if (plainPorts && (!Array.isArray(plainPorts) || !Array.isArray(tlsPorts))) {
+      throw new Error(".listen() must be used with plain and tls port arrays, like this: `.listen([80], [443, 5001], function () {})`");
+    }
+
     var results = {
       plainServers: []
     , tlsServers: []