[Plugin: Amcaptcha] Doesn’t work

“You forget to check «I’m not a spam bot» field to pass amcaptcha’s filter. Here is your comment (copy it to clipboard, just in case)”

field comes up when pressing “submit comment” regardless of having clicked the field (with resulting check appearing) or not

In IE8.0 and Mozilla 3.6

http://wordpress.org/extend/plugins/amcaptcha/

Same error occured both with the 1.6 and the new 1.7