A user reports on our list:

[sxin@goldstan ] $ curl -k -u admin:password -H "Accept: version=2,
application/json" -H "Content-Type: application/json" -X POST -d '{
"name":"goldtest.abc.com","hostgroup_id":3,"mac":"A0:48:1C:90:3E:5E" }'
https://foreman.abc.com/api/hosts
{
  "error": {"id":null,"errors":{"interfaces.mac":["can't be
blank"]},"full_messages":["Mac can't be blank"]}
}