diff --git a/lib/helper.js b/lib/helper.js
index 8e49885e..1fcb9dbf 100644
--- a/lib/helper.js
+++ b/lib/helper.js
@@ -158,18 +158,23 @@ class Helper {
      * @return {string}
      */
     function stringifyArgument(arg) {
-      if (typeof arg !== 'function') {
-        try {
-          return JSON.stringify(arg);
-        } catch (e) {
-          // The object was recursive
-          return arg.toString();
-        }
+      if (Helper.isString(arg) || Helper.isNumber(arg) || !arg)
+        return JSON.stringify(arg);
+      if (typeof arg === 'function') {
+        let text = arg.toString().split('\n').map(line => line.trim()).join('');
+        if (text.length > 20)
+          text = text.substring(0, 20) + '…';
+        return `"${text}"`;
       }
-      let text = arg.toString().split('\n').map(line => line.trim()).join('');
-      if (text.length > 20)
-        text = text.substring(0, 20) + '…';
-      return `"${text}"`;
+      const state = {};
+      const keys = Object.keys(arg);
+      for (let key of keys) {
+        const value = arg[key];
+        if (Helper.isString(value) || Helper.isNumber(value))
+          state[key] = JSON.stringify(value);
+      }
+      const name = arg.constructor.name === 'Object' ? '' : arg.constructor.name;
+      return name + JSON.stringify(state);
     }
   }