getting error: Warning: array_key_exists()

using the search form shortcode; clicking submit takes me to the event listings page just as it should.

However, on the page I get the following error:

Warning: array_key_exists() [function.array-key-exists]: The first argument should be either a string or an integer in C:\wamp\www\wp_\wp-content\plugins\events-manager\em-template-tags.php on line 280

http://wordpress.org/extend/plugins/events-manager/