sending smart transactional email results in CreateSend::ServerError #42

CreateSend::ServerError: CreateSend::ServerError
from /Users/*******/.rvm/gems/ruby-2.2.0/gems/createsend-4.1.0/lib/createsend/createsend.rb:283:in `handle_response'

class Indigo 
  require 'createsend'

  extend Formats

  API_KEY = "*********************"
  BOOKING_CONFIRMATION_ID = "028ec00a-e764-4291-a294-***********"

  def self.booking_confirmation(booking) 
    hashify_and_send_booking(BOOKING_CONFIRMATION_ID, booking) if booking
  end

  def self.hashify_and_send_booking(type, booking)
    smart_transactional_email(type).send(hashify_booking(booking)) 
  end

  private

  def self.hashify_booking(booking)
    h=Hash.new.tap do |ph|
      ph[:To] = [booking.firstname, "<#{booking.email}>"]
      ph[:Data] = Hash.new.tap do |dh|
        dh["firstName"] = booking.firstname
        dh["spaceTitle"] = booking.space_title
        dh["venueTitle"] = booking.venue_title
        dh["bookingDate"] = formatted_datetime booking.booked_from
        dh["bookingId"] = booking.id
      end
    end
    JSON.generate h      
  end

  def self.smart_transactional_email(type)
    CreateSend::Transactional::SmartEmail.new({api_key: API_KEY}, type)
  end

end