activeadmin: Testing active admin returns "undefined method `view_factory`"

I am using Spork, Guard, Capybara, and TestUnit.

When I try to do a simple test:

visit new_admin_user_session_path
fill_in 'admin_user[email]',    with: 'foobar@example.com'
fill_in 'admin_user[password]', with: 'password'
click_button 'Login'

I get the following error on the last line (click_button ‘Login’):

ActionView::Template::Error: undefined local variable or method `view_factory' for #<#<Class:0x000001070ccf80>:0x000001070c7648>

Please advise.