An strange error message

Hi:

I use visual studio 2012, and meet a problem when build and run my project. 

The error message shows it can not find AbbreviatedMonthNames, however, since it is an exception , 

the project can still run well , but it show this message every time after build and run.

The below is the code snippet. 

// Name:        MicrosoftAjax.debug.js
// Assembly:    System.Web.Extensions
// Version:     3.5.0.0
// FileVersion: 3.5.30729.5458
//-----------------------------------------------------------------------
// Copyright (C) Microsoft Corporation. All rights reserved.
//-----------------------------------------------------------------------
// MicrosoftAjax.js
// Microsoft AJAX Framework.

....
 function Sys$CultureInfo$_getAbbrMonthIndex(value) {
        if (!this._upperAbbrMonths) {
            this._upperAbbrMonths = this._toUpperArray(this.dateTimeFormat.AbbreviatedMonthNames);
        }
        return Array.indexOf(this._upperAbbrMonths, this._toUpper(value));
    }

What is is happen, and how to fix that. And I really don't know what is the AbbreviatedMonthNames, Thanks a lot.