Ajax.ActionLink Is Not Working - Response.IsAjaxRequest() Is Always False

I have been googling/SO:ing this issue for a while and many seem to be sharing this, but I haven't found any successful solution to my problem.

Using MVC3 and Razor.

Master page contains:
<script src="@Url.Content("~/Scripts/jquery-1.5.min.js")" type="text/javascript"></script>
<script src="@Url.Content("~/Scripts/MicrosoftAjax.js")" type="text/javascript"></script>
<script src="@Url.Content("~/Scripts/MicrosoftMvcAjax.js")" type="text/javascript"></script>
AjaxTest.cshtml contains:
<div id="AjaxTestDiv">content</div>
@Ajax.ActionLink("Update", "AjaxTester", new AjaxOptions { UpdateTargetId = "AjaxTestDiv" })
AjaxTester action method:
public string AjaxTester()
{
if (Request.IsAjaxRequest())
{
return DateTime.Now.ToString();
}
else
{
return "FAIL";
}
}

I always get the "FAIL" returned, to a blank page, not in the targeted div.

Edit: Also note that if I remove the if (Request.IsAjaxRequest()), I still don't get back anything to the targeted div, but instead a blank page.

Edit2: Looking at the HTML generated, this is my link:

<a data-ajax="true" data-ajax-method="POST" data-ajax-mode="replace"
data-ajax-update="#AjaxTestDiv" href="/Area/AjaxTester">Update</a>

Have tried switching the method to GET, to no avail.