Volume using variable with fallback defined in .env file stopped working in compose v2.

.env

VOLUME_SSH_AUTH_SOCK=${SSH_AUTH_SOCK:-/run/host-services/ssh-auth.sock}

docker-compose.yml

# ...
volumes:
# ...
  - ${VOLUME_SSH_AUTH_SOCK}:/ssh-agent

produces
error decoding 'Volumes[12]': invalid spec: :-/run/host-services/ssh-auth.sock}:/ssh-agent: empty section between colons

Works when using compose v1.
Works when using v2 only when the fallback of the VOLUME_SSH_AUTH_SOCKis not present or when the fallback is present in the compose file and not in the .env file.

ie.
docker-compose.yml

# ...
volumes:
# ...
  - ${SSH_AUTH_SOCK:-/run/host-services/ssh-auth.sock}:/ssh-agent

works in both v1 and v2