Response.Redirect acting wierd

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C#

ASP.NET

Javascript

Ajax

I have a login.aspx page with a textbox and button inside an UpdatePanel. I've wired a JS function to the textbox's onkeydown event so that if the user hits enter it will click the button

function clickButton(button)
{
    if ((event.which && event.which == 13) || (event.keyCode && event.keyCode == 13))
    {
        document.getElementById(button).click();

        return false;
    }
    else
    {
        return true;
    }
}

this works(the button's click event fires on the server). The click event checks the value in the textbox then does Response.Redirect("Default.aspx"); when that line executes the Login.aspx's Page_Load event fires again. I've stepped through this a few times and cant see why. However if a user clicks the button with the mouse everything works perfectly. I put breakpoint in Default's Page_Load and it never got hit so i'm pretty sure it isnt that page redirecting back to Login.

I'm obviously doing something wrong, but cant seem to figure out what it is or how to fix it. I've use the js function before and havent had any problems.

Any help would be appreciated.