Calling into a mocked collection?

I keep getting an exception in my target when trying to call into the collection to retrieve an item.

what might i be doing wrong?

here's the exception:

Microsoft.TeamFoundation.WorkItemTracking.Client.FieldCollection.GetById(Int32 id)
UpdateAssignee.Execute() in C:devTrimble.MobileSolutionscurrentuildTasksTeamBuildUpdateAssignee.cs: line 16
TeamBuild.Test.UpdateAssigneeTest.CallExecute() in C:devTrimble.MobileSolutionscurrentuildTasksTeamBuild.TestUpdateAssigneeTest.cs: line 49


here's the setup code:

UpdateAssignee target = this.GetUpdateAssignee();
UpdateAssignee_Accessor accessor = UpdateAssignee_Accessor.AttachShadow(target);

TeamFoundationServer mockedTfs = RecorderManager.CreateMockedObject<TeamFoundationServer>(Constructor.Mocked, "http://nowhere.com");
WorkItemStore mockedWis = RecorderManager.CreateMockedObject<WorkItemStore>(Constructor.Mocked, mockedTfs);
WorkItemType wit = RecorderManager.CreateMockedObject<WorkItemType>(Constructor.Mocked);
WorkItem mockedWi = RecorderManager.CreateMockedObject<WorkItem>(Constructor.Mocked, wit );
FieldCollection mockedFields = RecorderManager.CreateMockedObject<FieldCollection>(Constructor.Mocked );
Field mockedField = RecorderManager.CreateMockedObject<Field>(Constructor.Mocked);

// Start Mocking
using (RecordExpectations recorder = new RecordExpectations())
{
recorder.ExpectAndReturn(mockedTfs.GetService(typeof(WorkItemStore)), mockedWis).RepeatAlways();
recorder.ExpectAndReturn(mockedWis.GetWorkItem(target.WorkItemId), mockedWi).RepeatAlways();
recorder.ExpectAndReturn(mockedWi.Fields, mockedFields).RepeatAlways();
recorder.ExpectAndReturn(mockedFields.GetById((int)CoreField.AssignedTo), mockedField).RepeatAlways();
mockedField.Value = "Tim Bassett";

}