Undefined index _mbdb_isbn in other-schema.php

Hello,

Using WP 5.8.1, Mooberry Book Manager 4.9.9 and Schema & Structured Data for WP & AMP 1.9.85.

In frontend, URL https://www.domain.com/book/my-first-book/ I can see this warning:

Notice: Undefined index: _mbdb_isbn in /home/user/domain.com/wp-content/plugins/schema-and-structured-data-for-wp/output/other-schema.php on line 532

Debug mode ON.

Many thanks!