Using Geokit to return closest record returns PG::SyntaxError

I'm using Geokit to find the closest station to a location. The code runs after validation.

Here's the Location model:

class Location < ActiveRecord::Base
  belongs_to :station

  # Find nearest station after validation
  after_validation :update_nearest_station

  def update_nearest_station
    nearest_station = Station.closest(:origin => [latitude,longitude])

    update_attribute(:station_id, nearest_station.id)
  end
end

and the Station model:

class Station < ActiveRecord::Base
  has_many :locations

  acts_as_mappable :default_units => :km, :lat_column_name => :latitude, :lng_column_name => :longitude
end

When I run the code either by editing/adding a new Location or through my seed data I get the following error:

ActiveRecord::StatementInvalid: PG::SyntaxError: ERROR:  syntax error at or near ")"

LINE 4: ... SIN(0.8992918053979032)*SIN(RADIANS(stations.latitude))))*)
                                                                  ^
: SELECT  "stations".* FROM "stations"  ORDER BY 
      (ACOS(least(1,COS(0.8992918053979032)*COS(-0.0014597933863680574)*COS(RADIANS(stations.latitude))*COS(RADIANS(stations.longitude))+
      COS(0.8992918053979032)*SIN(-0.0014597933863680574)*COS(RADIANS(stations.latitude))*SIN(RADIANS(stations.longitude))+
      SIN(0.8992918053979032)*SIN(RADIANS(stations.latitude))))*)
      asc LIMIT 1

I've tried lots of different ways of doing this using Geokit's other methods (e.g. within with first) but I can't seem to get it to work.

Is this a bug with Geokit or am I missing something really obvious?

geokit / geokit-rails

Using Geokit to return closest record returns PG::SyntaxError #71