.NET: LINQ's Last()

I'm new to LINQ and LINQ to SQL and don't understand what's wrong with this code. The Excetpion.Message I get is

"Query operator 'Last' is not supported."

What I'm trying to do is get the earliest LastActivityUtcout of the latest 100. The code follows.

var postTimes = from post in db.Post
                where post.LastActivityUtc != null
                orderby post.LastActivityUtc descending
                select post.LastActivityUtc;

DateTime startDate = DateTime.MinValue;

if (postTimes.Count() >= 2)
{
    startDate = postTimes.Take(100).Last().Value;
}