Vaadin8 Combobox display key not in listing

Im’ using a Combobox binded to bean’s property “emailTemplateName” like

binder.forField(templateCbx).asRequired(reqMsg).bind("emailTemplateName"); Combobox is allowed on input text and empty selection too.
When I load the view, I don’t provide data to combobox, because is tied to another value still empty.
The displayed selected value in the combo is the one coming from bean, even if it is not present in items list.
Exploring Vaadin Combobox code, I found the missing key is created on the fly.
I think this is not correct.
Is there any way to avoid this? Shoul I configure combobox in some specific way?