Pick list choices / Enumeration constraints


I have an document type which has “String” fields.

Some of these fields have a content type to “String” and “enumeration” with the following list: “A”, " " (space) and “” (empty) (these constraints are imported from a xsd).

The fields are well setted to “A” or " " (space) if I manually select it. But for the “” (empty) value it doesn’t create/declare the field in the pipeline at all.
Obviously I would like the field to be declared…

The main problem happens here : I try to validate the document before converting it to an xml, and it fails because the field is not declared…

Any idea ?

(PS:on Wm8)