diff --git a/src/languages/javascript.js b/src/languages/javascript.js
index 18e8f75..16aa887 100644
--- a/src/languages/javascript.js
+++ b/src/languages/javascript.js
@@ -6,7 +6,7 @@ class JavaScript extends Language {
             highlight: 'js',
             aliases: ['javaScript', 'js'],
             options: {
-                harmony: () => true
+                harmony: () => ''
             }
         });
     }
diff --git a/src/listeners/messageInvalid.js b/src/listeners/messageInvalid.js
index cb830f5..306c6bd 100644
--- a/src/listeners/messageInvalid.js
+++ b/src/listeners/messageInvalid.js
@@ -76,9 +76,13 @@ class MessageInvalidListener extends Listener {
                 continue;
             }
 
-            const ok = Object.prototype.hasOwnProperty.call(language.options, key) && language.options[key](value);
-            if (ok) {
-                valid.set(key, value);
+            if (Object.prototype.hasOwnProperty.call(language.options, key)) {
+                const parse = language.options[key](value);
+                if (parse != null) {
+                    valid.set(key, parse);
+                } else {
+                    invalid.push(key);
+                }
             } else {
                 invalid.push(key);
             }