Error No NS record at zone apex in zone example.tld when modifying a DNS zone

Symptoms
The following error is thrown when modifying a DNS zone:
Error No NS record at zone apex in zone example.tld
Cause
There are no records of NS type in the zone.
If Openprovider nameservers are being used, these records are automatically added.
The error can occur when 'custom nameservers' / 'vanity nameservers' / 'whitelabled nameservers' functionality is enabled within a reseller account.
Resolution
Add NS records to the zone, which correspond to the nameservers that are assigned to the domain.