Validator not triggering

I have this drop down and required validator:

1

Here are the properties for the drop down:

And the validator:

3

When I try and save the record without an item selected, the validator doesn't trigger and we just get an exception back from the server because it invalidates the foreign key.

Can anyone see that I'm doing something wrong?

Thanks.

What is the type of the ProductLineId property? The required validator will trigger at the moment only if the default value is null. If the property is not nullable its default value will be 0 which is currently valid from the required validator's point of view.