Silverlight MonthPlanner Control

For the first time since I bought the product I'm trying to use the MonthPlanner control.  (Note: the DayPlanner control has proved really useful for us!) 

But, when I attempt to bind the data records to the control, I keep getting a just-in-time debugger error stating that the following method had a null reference exception:
However, I cannot figure out what is causing this error.   The XAML for the planner control is:


my:MonthPlanner x:Name="TMSPlanner" VerticalAlignment="Top" HorizontalAlignment="Left" Margin="0,0,0,0" Height="500" Width="800" DisplayFormat="dd/MM/yyyy" ShowToday="True" BindingValidationError="TMSPlanner_BindingValidationError" Refreshed="TMSPlanner_Refreshed" ItemRetrieved="TMSPlanner_ItemRetrieved" ItemsRetrieved="TMSPlanner_ItemsRetrieved" CaptionContent="{StaticResource CaptionContentDataTemplatePreview}" ItemContent="{StaticResource ItemDataTemplate}" HeaderBackBrush="{StaticResource PlannerHeaderBrush}" Background="{StaticResource PlannerBackgroundBrush}" SelectionBrush="{StaticResource PlannerSelectedCell}" > </my:MonthPlanner>
The error continues to happen even if I remove the Refreshed, ItemRetrieved, ItemsRetrieved events plust the CaptionContent, ItemContent, HeaderBackBrush, Background, and SelectionBrush styles.
When the form is loaded, I instantiate a databinding class and set it to the databinding property, where _paramPlannerAssignments has the starting list of data items to show and _paramStartDate has the month, such as 2/1/2012:


TMSPlanner.StartMonth = _paramStartDate.Value;

TMSPlanner.StartTime = _paramStartDate.Value;

_localPlanner = new Components.MyDatabindingClass(_paramPlannerAssignments);

TMSPlanner.Databinding = _localPlanner;



How can I find out what is causing this runtime JIT error?  The code all works without exceptions, but when the form is finally rendered, the JIT error happens.  When running it outside of Visual Studio, the JIT error does not come up, but the planner appears empty despite the starting item collection.
I've looked at the demos but cannot find any oddities in my code or XAML when I compare the two.
Thanks for any insights you might be able to provide me with to help me identify the source of this most frustrating error.

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