When running on a Samba 4.7.7 ROLE_DOMAIN_PDC, the "testparm" tool outputs the following:
idmap range not specified for domain '*'
ERROR: Invalid idmap range for domain *!
This is obviously targeted to a domain member and should not occur on a PDC.
A check should be included in the tool to verify which server role it is being running on. See the resolved bug 12629 for the case of Samba as AD DC.