PageMethods and masterpage troble

Hi. I have such problem.I try to invoke [webmethod] with help of javascript.But on masterpage it didn't work.I get an error "PageMethods is undefined".Could someone write me  workable example how can i add  a [webmethod] to the masterpage  and invoke it with help of javascript?
example:
here is my code behind file it is in masterpage.cs: 
        [WebMethod()]
        public static string SomeFunc()
        {
            return "";
        }
this function should invoke from masterpagefile aspx file:
<script type="text/javascript" language="javascript">
function invoke()
{   
    PageMethods.SomeFunc();
}
</script>
But it didnt work in ...