Castle ActiveRecord: Save is throwing StaleStateException

Question

Following this question, I tried to convert a sequence of try..Insert()..catch..Update() into a single call to Save(). I'm getting StaleStateException with "Unexpected row count: 0; expected: 1".

What am I doing wrong?

Here is the code that fails:

var u = new User {SignupDate = DateTime.Now, Name="Foo", OpenId = "Bar"};
ActiveRecordMediator<User>.Save(u);
var q = new Question
  {
    Author = u,
    Body = "",
    CreatedDate = DateTime.Now,
    LastRelatedUser = u,
    Title = "",
    UpdateDate = DateTime.Now
  };

ActiveRecordMediator<Question>.Save(q);
var v = new VoteOnQuestion {Key = new VoteKey(u.Id, q.Id)};

// This fails with StaleStateException
ActiveRecordMediator<VoteOnQuestion>.Save(v);

VoteOnQuestion class simply inherits from VoteOnPost, which looks like this:

public abstract class VoteOnPost
{
    [CompositeKey]
    public VoteKey Key { get; set; }

    [Property]
    public VoteType Vote { get; set; }
}