Shortcode and Filterform Error

This
[events_filterform notcategory=10 multiple=1]
gives me, when used in the front end, this
Warning: strpos() expects parameter 1 to be string, array given in /www/htdocs/wxxx/wp-includes/shortcodes.php on line 386.

What am I doing wrong?

https://wordpress.org/plugins/events-made-easy/