rake db:migrate problem

I am just starting out learning RoR. I am trying to follow along with
the Depot project in "Agile Web Development With Rails ed2" and get the
error "You have a nil object when you didn't expect it! You might have
expected an instance of Array. The error occured while evaluating
nil.[]" I ran --trace and get the following, but I am not sure what it
means. Can anyone help me make sense out of it? Thanks

Mike Rensing

(in /Users/mike/work/depot)
** Invoke db:migrate (first_time)
** Invoke environment (first_time)
** Execute environment
** Execute db:migrate
== AddPrice: migrating

-- add_column(:products, :price, :decimal, {:default=>0, :precision=>8,
:scale=>2})
rake aborted!
You have a nil object when you didn't expect it!