When I try to modify and then save a model using DataMapper I get a SaveFailure exception but no errors.

Specifically I see this message: "MonthlyBill#save returned false, MonthlyBill was not saved"

This is the code doing the saving:

post '/monthly_bills' do
  with_authenticated_user do |user|
  description = params[:description]
  expected_amount = params[:expected_amount]
  pay_period = params[:pay_period]

  monthly_bill = MonthlyBill.new(:description=>description, :expected_amount=>expected_amount, :pay_period=>pay_period)
  user.MonthlyBills << monthly_bill
  user.save
end

The User model:

class User
  include DataMapper::Resource

  property :id,             Serial
  property :email_address,  String
  property :password,       String

  has n, :MonthlyBills
  has 1, :CurrentPayPeriod
end

The MonthlyBill model:

class MonthlyBill
  include DataMapper::Resource

  property :id,             Serial
  property :description,    String
  property :expected_amount,Decimal
  property :pay_period,     Integer

  belongs_to :user
end

What is the issue and, more importantly, how can I get DataMapper to tell me more specifically what is wrong?