CiviCRM issue when subscribing on behalf of an organisation with 4.7.3 and 4.7.4 releases
I changed it to be 'optional' and it worked for me. http://dmaster.demo.civicrm.org/civicrm/contact/view?reset=1&amp;cid=225

Can you confirm this is the issue?

I  feel like this has come up recently but don't have time to search SE or JIRA - can you try and track down if an issue exists, or if one has been 'fixed' recently, then reopen it?