OJS XML Issue on with Datacite registration

sending on datacite the registration for an article with OJS export plug in I receive this error:

Registration was not successful! The DOI registration server returned an error: ‘400 - [xml] xml error: cvc-minLength-valid: Value ‘’ with length = ‘0’ is not facet-valid with respect to minLength ‘1’ for type ‘nonemptycontentStringType’.’.

It seems that there is an uncompiled field.

But I can’t find any info about it. Can you help me?