[Plugin: WP Events Calendar] Call to undefined function get_the_post_thumbnail()

Hi all,

I’m working on this plugin, and at the moment im working on the thickbox popup. I have to modify ec_day.class.php which contains the display function. If i use get_post(), all works with no problems. But if i use get_the_post_thumbnail(), i receive

Fatal error: Call to undefined function get_the_post_thumbnail() in /web/htdocs/www.jewishcommunityofrome.com/home/wp-content/plugins/events-calendar/ec_day.class.php on line 94

Of course, i have in my functions.php

add_theme_support( 'post-thumbnails' );