nbd-server doesn't start with ipv6.disable=1 and unplugged cable

[alkisg]

When the following two conditions are true, nbd-server doesn't start:

• ipv6.disable=1 exists in /proc/cmdline
• the network cable is unplugged, so that eth0 doesn't have an ipv4 IP at boot either

The error message in the logs is:

Jan 31 13:29:33 srv1-dide nbd_server[1330]: failed to setup servers: failed to open a modern socket: failed to create a socket: Address family not supported by protocol

I tested on Ubuntu 16.04 (nbd-server 1:3.13-1) and on Ubuntu 18.04 (nbd-server 1:3.16.2-1).