Exception is Not Being Caught by ObjectDatasource Control

Hi,
I am using the entity framework tutorials by Tom Dykstra to create a small web application (Using the Entity Framework and the ObjectDataSource Control, Part 2: Adding a Business Logic Layer and Unit Tests ).  I am at the at the stage of adding Business Logic.  I have an update method which throws an exception.  I have an Object Datasource control and I am using this update method to update a person.  When the person is updated there is a check to see if a condition passes or fails.  When the condition fails an exception is thrown.  I have another method in my code behind which handles this exception if it is caught by the Object Datasource Control.  This is setup on the OnUpdate event. 
I can see that if no exception is thrown the OnUpdate method is called and nothing happens.  This is correct.
However, if the exception is thrown I get an error saying that the Exception Was Unhandled by user code.&n ...