MVC :: Check If Record Exists Before Insert In 2.0 Using Ajax

I want to check if the record is already exists before insert using Ajax ActionLink in Create View. I am getting error 'Object reference not set to an instance of an object.' Here is a code in View :

<div class="editor-label">
<%= Html.LabelFor(model => model.PolicyId) %>
</div>
<div class="editor-field">
<%= Html.TextBoxFor(model => model.PolicyId) %>
<%= Html.ValidationMessageFor(model => model.PolicyId) %>
<em>can not be changed later.</em>
//error occurs here
<%= Ajax.ActionLink("Check", "CheckIfExists", "Life", new { PolicyId = Model.PolicyId }, null)%>
</div>

And Controller action :

public JavaScriptResult CheckIfExists(string PolicyId)
{
if (lifeRepository.IsExists(PolicyId))
return JavaScript("alert(' Already Exists');");
else
return JavaScript("alert(' Not Exists');");
}