cannot redirect to other pages in asp.net

[tisa]

hello,

I have a question about the redirect function in javascript

in code behind:
i have a link button called btnl_logout

btnl_logout.Attributes.Add("onclick", "confirm_logout();")

after click this link, the confirm_logout function will called as following

<script language="javascript">
function confirm_logout()
{
if (confirm("Are you sure you want to logout?")==true)
{
document.location ="http://hk.yahoo.com";
return;
}
else
{
alert("a");
return false;
}
}
</script>
even i change many times in document.location's value, but still cannot redirect,
why??

Please help~~