Not possible to activate user account when username contains Cyrillic letters

Steps to reproduce:

1. Go to http://qa.jtalks.org/antarcticle/ and press Sign Up button.
2. Fill out "Email" and "Password" fields with the valid data.
3. Fill out "Username" field with valid data that contains Cyrillic letter.
4. Check your email for activation letter and follow the activation link in this letter.

Actual result:
User is redirected to Antarcticle Error 500 page. User account is not activated.
Expected result:
User is redirected to Antarcticle main page and is signed in. User account is activated.