diff --git a/test/built-ins/String/prototype/concat/S15.5.4.6_A1_T10.js b/test/built-ins/String/prototype/concat/S15.5.4.6_A1_T10.js
index 5f212d16db..66d9f49789 100644
--- a/test/built-ins/String/prototype/concat/S15.5.4.6_A1_T10.js
+++ b/test/built-ins/String/prototype/concat/S15.5.4.6_A1_T10.js
@@ -7,7 +7,7 @@ es5id: 15.5.4.6_A1_T10
 description: Call concat([,[...]]) function with object arguments
 ---*/
 
-var __obj = {toString:function(){return "\u0041";}}
+var __obj = {toString:function(){return "\u0041";}, valueOf:function(){return "_\u0041_";}}
 var __obj2 = {toString:function(){return true;}}
 var __obj3 = {toString:function(){return 42;}}
 var __str = "lego";
@@ -15,8 +15,11 @@ var __str = "lego";
 //////////////////////////////////////////////////////////////////////////////
 //CHECK#1
 with(__str){
+    if (concat(__obj) !== "legoA") {
+      $ERROR('#1: var x; var __obj = {toString:function(){return "\u0041";}, valueOf:function(){return "_\u0041_";}}; var __str = "lego"; concat(__obj) === "legoA". Actual: '+concat(__obj) );
+    }
     if (concat(__obj, __obj2, __obj3, x) !== "legoAtrue42undefined") {
-      $ERROR('#1: var x; var __obj = {toString:function(){return "\u0041";}}; var __obj2 = {toString:function(){return true;}}; var __obj3 = {toString:function(){return 42;}}; var __str = "lego"; concat(__obj, __obj2, __obj3, x) === "legoAtrue42undefined". Actual: '+concat(__obj, __obj2, __obj3, x) ); 
+      $ERROR('#2: var x; var __obj = {toString:function(){return "\u0041";}, valueOf:function(){return "_\u0041_";}}; var __obj2 = {toString:function(){return true;}}; var __obj3 = {toString:function(){return 42;}}; var __str = "lego"; concat(__obj, __obj2, __obj3, x) === "legoAtrue42undefined". Actual: '+concat(__obj, __obj2, __obj3, x) ); 
     }
 }
 //