This code is complaining: failed with error "not master"

"Activity" is my model.

Activity.mongo_session.with(consistency: :strong).
  command(create: 'my_capped_collection', capped: true, size: 7500000, max: 100000)

I'm running mongoid from master at f89502d7d0b418c20f5b04f5d106b61b4091cd68
and Rails 4.