Are you presenting this list in the content outline (as opposed to as a property editor in a section)? Could you try placing it in a section to see if you get the validation message? It is possible that content outline presentation doesn't surface all validation messages.
If you are looking at min=1/max=1 scenario, perhaps you need an ElementProperty rather than a ListProperty or maybe even ImpliedElementProperty.
An ImpliedElementProperty lets you have a child element in the model that you don't need to create explicitly. Whenever the child element's properties are set, the XML element associated with the implied element is created as well.
An ElementProperty gives you explicit control over when the child element and the corresponding XML element is created.
There are examples of both in the samples project. See the contacts sample in particular.