I'm attempting a buildInfo: 1 command on a replica set (I'm specifying two members when connecting) and I get:

NoMethodError: private method `select' called for nil:NilClass
  from [...]/lib/mongo/server_selector/selectable.rb:138:in `primary'
  from [...]/lib/mongo/server_selector/primary_preferred.rb:89:in `select'
  from [...]/lib/mongo/server_selector/selectable.rb:94:in `select_server'
  from [...]/lib/mongo/database.rb:130:in `command'

I would expect :primary_preferred to use the secondary when the primary fails. This just times out after the specified :socket_timeout.

In any case, it should at least return an error I can handle more gracefully (one that inherits Mongo::Error)