[Validation series] Validator.ShowErrorMessageText crashing on bp:DropDownList control

So I have this ddl:

The property bound to SelectedValue is this:

And then I hit a button that has Validation.Enabled="false" on it, that is outside the viewmodel in which resides the ddl and any button that could trigger the validation… And I get this crash instead of an open ModalDialog:

My questions are:

  • if Validator.ShowErrorMessageText can be used with bp:DropDownList, what am I doing wrong?
  • if it cannot be used with bp:DropDownList, why is the markup editor suggesting it to me in autocomplete?