Setting EnablePageMethods to true, generates "Could not load type 'System.Web.Configuratiobn.ScriptingJsonSerializationSection".

When I turn on EnablePageMethods="true"  I get error

 <asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="true">      

The error I get is:
Description:
An error occurred during the processing of a configuration file required to
service this request. Please review the specific error details below and modify
your configuration file appropriately.

Parser Error Message: An
error occurred creating the configuration section handler for
system.web.extensions/scripting/webServices/jsonSerialization: Could not load
type 'System.Web.Configuratiobn.ScriptingJsonSerializationSection' from assembly
'System.Web.Extensions, Version=1.0.61025.0, Culture=neutral,
PublicKeyToken=31bf3856ad364e35'.< ...