ActiveRecord AssociationType Mismatch

I have a Rails 3.2.21 app where I'm adding a simple form/collection_select field in my form. When selecting an object from the dropdown (or leaving it blank) I get the following error:

ActiveRecord::AssociationTypeMismatch at /calls/5
BillFacility(#70179530126460) expected, got String(#70179505820040)

Here's what my models look like:

call.rb

belongs_to :bill_facility
attr_accessible :bill_facility_id

bill_facility.rb

has_many :calls

Here's what my form looks like:

_form.html.erb

<%= f.collection_select(:bill_facility_id, BillFacility.order("facility_name ASC"), :id, :facility_name, {:include_blank => true}, {:class => 'select'}) %>

Here's the migrations I did to add the BillFacility model and add the bill_facility_id to the Call model:

class CreateBillFacilities < ActiveRecord::Migration
  def change
    create_table :bill_facilities do |t|
      t.string :facility_name
      t.string :facility_address

      t.timestamps
    end
  end
end

class AddBillFaciltyIdToCalls < ActiveRecord::Migration
  def change
    add_column :calls, :bill_facility_id, :integer
  end
end

If I manually assign the call an id in bill_facility_id I get an unknown to_i method error. If I manually make it nil then select a BillFacilty from the drop down (or leave it blank) I get the mismatch error:

ActiveRecord::AssociationTypeMismatch at /calls/5
BillFacility(#70179530126460) expected, got String(#70179505820040)

I'm sure this is something simple that I'm missing. Anyone have any ideas on what I'm doing wrong here? It's a bit early so my mind is kind of fuzzy so I should probably wait until I'm fully awake but figured I'd ask for some help.

If you have any questions, please let me know.