Sometimes it is necessary to mix json.js functions like toJSONString() with client script generated by the AJAX.NET library.

If you include the json.js file in your client script you might receive enumValueNotInteger error.

A little digging around landed me on the following helpful discussion at http://forums.asp.net/t/1077290.aspx

Steve Marx raises several options to solve the problem in the above discussion.

To solve my problem I used the script manager Scripts property option.

Example:

<ajaxToolkit:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">

<Scripts>

   <asp:ScriptReference Path="json.js" />

</Scripts>

</ajaxToolkit:ToolkitScriptManager>

Note if you are using master pages or need json.js script inside custom controls you can use the script manager proxy Scripts property instead.

Example:

<asp:ScriptManagerProxy ID="ScriptManagerProxy1" runat="server">

<Scripts>

   <asp:ScriptReference Path="json.js" />

</Scripts>

</asp:ScriptManagerProxy>