sn method does not work

[bruno-]

Hi,
for some reason sn nor serial_number methods do not work. Here is the blueprints file:

require 'machinist/active_record'
require 'sham'
require 'faker'

Device.blueprint do
end

Device.blueprint(:enabled) do
token { "RAND-#{sn}" } # generates 4 random digits
last_checkin_time { Time.now }
end

Here is the error I get when running cucumber scenario:

@javascript
Scenario: enabled device is shown in system status # features/dashboard.feature:33
Given there is a device (enabled) # cucumber_factory-1.8.3/lib/cucumber/factory.rb:24
No sham defined for sn (RuntimeError)
./spec/support/blueprints.rb:9:in block (2 levels) in <top (required)>' ./spec/support/blueprints.rb:9:inblock in <top (required)>'
features/dashboard.feature:34:in `Given there is a device (enabled)'

Any suggestions, why is this happening?