Captcha field isn't cleaned after incorrect input

Steps to reproduce
1. Open link http://uat.jtalks.org/jcommune/user/new
2. Fill all fields except captcha with correct data
3. Fill captcha field with incorrect data
4. Press button "Sign in"

Actual result
User receives error about incorrect captcha and captcha field isn't cleaned

Expected result
User receives error about incorrect captcha and captcha field has been cleaned