[Plugin: Enhanced Search Form] Filtered search not working!

On advanced search after search results pops out, we get the following error:

Warning: strtolower() expects parameter 1 to be string, array given in /var/www/vhosts/SilviaSandino.com/httpdocs/radio/wp-content/plugins/enhanced-search-form/enhanced-search-form.php on line 33 Warning: strtolower() expects parameter 1 to be string, array given in /var/www/vhosts/SilviaSandino.com/httpdocs/radio/wp-content/plugins/enhanced-search-form/enhanced-search-form.php on line 34

https://www.ads-software.com/extend/plugins/enhanced-search-form/