On Sign up popup window cursor is not focused on "refresh captcha" via Tab

Steps to reproduce
1. On main forum page click on "Sign up"
2. In "Sign up" popup window press Tab to move cursor on all fields and buttons

Actual result: Cursor focused on fields "Username", "Email", "Password", "Confirm password", "Captcha text" and button "Sign up" but cursor is not focused on "Captcha refresh" button
Expected result: Cursor focused on all fields and buttons: "Username" -> "Email" -> "Password" -> "Confirm password" -> "Captcha refresh" -> "Captcha text" -> "Sign up"