An error of type E_ERROR was caused in line 99 of the file

This happens when clicking on Tools-Site Health.

I’ve tried this on 2 different sites now and the same problem occurs.

Site Health says there has been a critical error but this only affects the Site Health screen, The rest of the site is functioning fine.

Error Details
=============
An error of type E_ERROR was caused in line 99 of the file /home/utag/public_html/wp-content/plugins/postmark-approved-wordpress-plugin/includes/class-postmark-debug.php. Error message: Uncaught TypeError: Postmark_Debug::strbool(): Argument #1 ($value) must be of type bool, null given, called in /home/utag/public_html/wp-content/plugins/postmark-approved-wordpress-plugin/includes/class-postmark-debug.php on line 64 and defined in /home/utag/public_html/wp-content/plugins/postmark-approved-wordpress-plugin/includes/class-postmark-debug.php:99
Stack trace:
#0 /home/utag/public_html/wp-content/plugins/postmark-approved-wordpress-plugin/includes/class-postmark-debug.php(64): Postmark_Debug->strbool()
#1 /home/utag/public_html/wp-includes/class-wp-hook.php(307): Postmark_Debug->debug_info()
#2 /home/utag/public_html/wp-includes/plugin.php(191): WP_Hook->apply_filters()
#3 /home/utag/public_html/wp-admin/includes/class-wp-debug-data.php(1461): apply_filters()
#4 /home/utag/public_html/wp-admin/site-health-info.php(32): WP_Debug_Data::debug_data()
#5 /home/utag/public_html/wp-admin/includes/class-wp-site-health.php(66): require_once(‘/home/utag/publ…’)
#6 /home/utag/public_html/wp-includes/class-wp-hook.php(307): WP_Site_Health->show_site_health_tab()
#7 /home/utag/public_html/wp-includes/class-wp-hook.php(331): WP_Hook->apply_filters()
#8 /home/utag/public_html/wp-includes/plugin.php(476): WP_Hook->do_action()
#9 /home/utag/public_html/wp-admin/site-health.php(205): do_action()
#10 {main}
thrown

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