coredns upstream config not allow blank

[panpan0000]

when no nameserver filled in host machines' /etc/resolv.conf coredns will fail to boot up with error plugin/forward: no nameservers found

I guess sth wrong in coredns config

kubectl edit configmap coredns -n kube-system

        forward . /etc/resolv.conf {
          prefer_udp
          max_concurrent 1000
        }