The remote name server is affected by a vulnerability.
ISC BIND versions 9.14.6 / 9.15.4 and earlier are affected by multiple vulnerabilities. - A flaw in mirror zone validity checking. It can allow zone data to be spoofed. (CVE-2019-6475) - A defect in code added to support QNAME minimization can cause named to exit with an assertion failure if a forwarder returns a referral rather than resolving the query. (CVE-2019-6476)
Upgrade to ISC BIND version 9.14.7 / 9.15.5 or later.