Comma gem export failing on Heroku - working on local environment

Comma gem export failing on Heroku - working on local environment

Using the the comma gem and exporting CSV files perfectly fine on my local
environment. Also working on our staging environment which is running on
an AWS instance.
ONLY failing on Heroku.
Heroku says:
2013-08-21T21:55:34.875724+00:00 app[web.1]: NoMethodError (undefined
method `klass' for nil:NilClass):
2013-08-21T21:55:34.875724+00:00 app[web.1]: app/models/job.rb:220:in
`block in <class:Job>'
2013-08-21T21:55:34.875724+00:00 app[web.1]:
app/controllers/jobs_controller.rb:25:in `block (2 levels) in index'
2013-08-21T21:55:34.875724+00:00 app[web.1]:
app/controllers/jobs_controller.rb:18:in `index'
And line 220 in my Job model is:
Comma do
...
customer :first_name
...
end
job.rb includes the line:
belongs_to :customer
And customer.rb has:
has_many :jobs
And as I mentioned this entire thing works great on my local box - I click
the export button and out pops all the jobs, with each customer in the
appropriate column.
For whatever reason it's only failing on Heroku.
I'm using Ruby 2.0.0-p0 on Heroku, Comma gem only verified up to 1.9.2
according to the documentation - BUT - I'm using Ruby 2.0.0-p0 on the
staging server AND on my local environment and, again, it's working.
It's the 'klass" that's really throwing me off, because I don't have that
anywhere in my code. I've read other threads about the same error and
their solutions, but I don't have things like nested model forms and the
like.
It is some issue with Heroku & Ruby 2.0 and associations? I've got the
has_many and belongs_to on both sides, but still seems to be
misunderstanding the association...