Uncaught TypeError: array_merge()

We are seeing this fatal on multiple sites after 3.6.8 update:

PHP Fatal error:  Uncaught TypeError: array_merge(): Argument #2 must be of type array, null given in /srv/htdocs/wp-content/plugins/my-calendar/my-calendar-output.php:1971
Stack trace:
#0 /srv/htdocs/wp-content/plugins/my-calendar/my-calendar-output.php(1971): array_merge(Array, NULL)
#1 /srv/htdocs/wp-content/plugins/my-calendar/my-calendar-shortcodes.php(102): my_calendar(Array)
#2 /wordpress/core/6.8.2/wp-includes/shortcodes.php(434): my_calendar_insert(Array, '', 'my_calendar')
#3 [internal function]: do_shortcode_tag(Array)
#4 /wordpress/core/6.8.2/wp-includes/shortcodes.php(273): preg_replace_callback('/\\[(\\[?)(my_cal...', 'do_shortcode_ta...', '[my_calendar id...')
#5 /wordpress/core/6.8.2/wp-includes/class-wp-hook.php(324): do_shortcode('[my_calendar id...')
#6 /wordpress/core/6.8.2/wp-includes/plugin.php(205): WP_Hook->apply_filters('[my_calendar id...', Array)
#7 /wordpress/core/6.8.2/wp-includes/post-template.php(256): apply_filters('the_content', '<p>[my_calendar...')
#8 /srv/htdocs/wp-content/themes/twentyten/loop-page.php(32): the_content()
#9 /wordpress/core/6.8.2/wp-includes/template.php(812): require('/srv/htdocs/wp-...')
#10 /wordpress/core/6.8.2/wp-includes/template.php(745): load_template('/srv/htdocs/wp-...', false, Array)
#11 /wordpress/core/6.8.2/wp-includes/general-template.php(206): locate_template(Array, true, false, Array)
#12 /srv/htdocs/wp-content/themes/twentyten/page.php(26): get_template_part('loop', 'page')
#13 /wordpress/core/6.8.2/wp-includes/template-loader.php(106): include('/srv/htdocs/wp-...')
#14 /wordpress/core/6.8.2/wp-blog-header.php(19): require_once('/wordpress/core...')
#15 /wordpress/core/6.8.2/index.php(17): require('/wordpress/core...')
#16 {main}
  thrown in /srv/htdocs/wp-content/plugins/my-calendar/my-calendar-output.php on line 1971