[Plugin: Postmark Email for WordPress by Yoast] Fatal error when using plugin with WooCommerce

I’m developing a new site with WooCommerce and new customer emails produce a fatal error when using the plugin. Test emails sent using Test Email plugin work fine. I changed line 79 of postmark.php to:

if ( !is_array($attachments) && !empty($attachments) )

This solves the issue.

https://www.ads-software.com/extend/plugins/postmark-email-for-wordpress/