PageMethods is undefined ...

I don't see why this happens! PageMethods are enabled

       <asp:ScriptManager ID="scriptManager" runat="server" EnablePartialRendering="True" EnablePageMethods="True">
            <Scripts>
                <asp:ScriptReference Path="js/file1.js" />
                <asp:ScriptReference Path="js/file2.js" />
            </Scripts>
        </asp:ScriptManager>
Codebehind
    [ScriptMethod]
    [WebMethod]
    public static XmlNode HelloWorld(string filter)
&nb ...