Microsoft JScript runtime error: 'PageMethods' is undefined

Hi

im getting the error

Microsoft JScript runtime error: 'PageMethods' is undefined

here is my cide

[WebMethod]
    public static bool CheckAvailability(string uid)
    {
    MembershipUser user = Membership.GetUser(uid);
    if (user == null)
    {
    return true;
    }
    else
    {
    return false;
    }
    }

and here is javascrript

<script type="text/javascript">
            function CheckAvailability() {
                var uid = $("#username").val();
                if (uid == "") {
                    alert('Please enter user ID!');
                    return false;
                }
                PageMethods.CheckAvailability(uid, OnComplete);
                return false;
            }

            function OnComplete(result) {
                var lblmsg = $("#lblmsg").val();
                if (result) {
                    lblmsg.innerText = "The ID is available!";
                }
                else {
                    lblmsg.innerText = "The ID is unavailable!";
                }
            }</script>

I have script manage  in master page file

<asp:ScriptManager ID="ScriptManager2" runat="server" EnablePageMethods="true" EnablePartialRendering="true" ></asp:ScriptManager>

I dont know if i have called javascript in wrong place on content page,

but simple alert message is working on that script

here in case      alert('Please enter user ID!');

Any help would be appreciated!!!

Thanks