Shoulda Matchers: undefined method `is_at_least' for :last_name:Symbol

I am trying to validate the length of first_name:

 it { should ensure_length_of (:first_name).is_at_least(2)}

But getting the error:

Shoulda Matchers: undefined method `is_at_least' for :last_name:Symbol

I can't find much in the docs on proper syntax in this situation.