Interaction w/custom publishing options: Array to string conversion in subscriptions_content_form_alter() [#2821250]

There appears to be a conflict between Subscriptions 7.x-1.3 and Custom Publishing Options 7.x-1.4.

When enabled, the subscriptions content module
1) throws an error for every custom publishing option set in the admin/structure/custom_pub config page (below), and
2) in the node edit form publishing options tab custom publishing options are not saved

Notice: Array to string conversion in subscriptions_content_form_alter() (line 502 of /srv/bindings/8418b541e2fa47b5b2e40b2fec0fb25c/code/sites/all/modules/subscriptions/subscriptions_content.module).