unbound dies on reboot : buffer overflow detected

[Sean Darcy]

1.8.1
When rebooting, unbound dies:

systemd[1]: Starting Unbound recursive Domain Name Server...
unbound-checkconf[1025]: unbound-checkconf: no errors in
/etc/unbound/unbound.conf
systemd[1]: Started Unbound recursive Domain Name Server.
unbound[1261]: [1261:0] notice: init module 0: ipsecmod
unbound[1261]: [1261:0] notice: init module 1: validator
unbound[1261]: [1261:0] notice: init module 2: iterator
unbound[1261]: [1261:0] info: start of service (unbound 1.8.1).
unbound[1261]: [1261:0] info: generate keytag query _ta-4a5c-4f66. NULL IN
unbound[1261]: [1261:2] info: generate keytag query _ta-4a5c-4f66. NULL IN
unbound[1261]: [1261:0] error: .: failed lookup, cannot transfer from
master k.root-servers.net
unbound[1261]: [1261:0] error: .: failed lookup, cannot transfer from
master k.root-servers.net
unbound[1261]: [1261:0] error: .: failed lookup, cannot transfer from
master g.root-servers.net
unbound[1261]: [1261:0] error: .: failed lookup, cannot transfer from
master f.root-servers.net
unbound[1261]: [1261:0] error: .: failed lookup, cannot transfer from
master e.root-servers.net
unbound[1261]: [1261:0] error: .: failed lookup, cannot transfer from
master c.root-servers.net
unbound[1261]: [1261:0] error: .: failed lookup, cannot transfer from
master b.root-servers.net
unbound[1261]: *** buffer overflow detected ***: /usr/sbin/unbound terminated
systemd[1]: unbound.service: Main process exited, code=killed, status=6/ABRT
systemd[1]: unbound.service: Failed with result 'signal'.

But no problem if restarted.
systemctl restart unbound

systemd[1]: Starting Unbound recursive Domain Name Server...
unbound-checkconf[2775]: unbound-checkconf: no errors in
/etc/unbound/unbound.conf
systemd[1]: Started Unbound recursive Domain Name Server.
unbound[2778]: [2778:0] warning: did not exit gracefully last time (1261)
unbound[2778]: [2778:0] notice: init module 0: ipsecmod
unbound[2778]: [2778:0] notice: init module 1: validator
unbound[2778]: [2778:0] notice: init module 2: iterator
unbound[2778]: [2778:0] info: start of service (unbound 1.8.1).
unbound[2778]: [2778:0] info: generate keytag query _ta-4a5c-4f66. NULL IN

Any help appreciated.

sean