From description for #map_reduce method (https://github.com/mongodb/mongo-ruby-driver/blob/master/lib/mongo/collection.rb#L581):

@option opts [Array] :sort ([]) an array of [key, direction] pairs to sort by. Direction should
be specified as Mongo::ASCENDING (or :ascending / :asc) or Mongo::DESCENDING (or :descending / :desc)

Actual is needed to do Mongo::Support.format_order_clause for this option.

I.e {:sort => [[:position, :asc]]} raises error, but {:sort => Mongo::Support.format_order_clause([[:position, :asc]])} executes appropriate map/reduce command.