ASP.Net AJAX Tab Control: clientActiveTabChanged is undefined

i have a tab conatiner with 5 tabs

i need to get the active tab index

so i have a javascript to get teh active tabindex but i get error

below is the runtime code viewd in browser

Sys.Application.add_init(function() {
    $create(AjaxControlToolkit.TabContainer, {"activeTabIndex":0,"autoPostBackId":"ctl00$ContentMain$Tabs","clientStateField":$get("ctl00_ContentMain_Tabs_ClientState")}, {"activeTabChanged":clientActiveTabChanged}, null, $get("ctl00_ContentMain_Tabs"));

});




function clientActiveTabChanged(sender, args) 
    {
       document.getElementById('<%=Htabindex.ClientID%>').value=sender.get_activeTabIndex();  
   
    }