ruby-on-rails - 「1」の未定義のメソッド「each」：文字列

Question

Rails モデルに属性を割り当てようとすると、奇妙な文字列エラーが発生します。

# POST /apartments
  # POST /apartments.xml
  def create
    @apartment = Apartment.new(params[:apartment])

パラメータ

{"utf8"=>"✓",
 "authenticity_token"=>"Kxxxxx=",
 "apartment"=>{"street"=>"123 Main St",
 "unit"=>"",
 "hide_address"=>"0",
 "city"=>"White Plains",
 "state"=>"New York",
 "zipcode"=>"10840",
 "country"=>"United States",
 "price"=>"4000",
 "security_deposit"=>"",
 "application_fee"=>"",
 "bedrooms"=>"4",
 "bathrooms"=>"3",
 "size"=>"",
 "available_date"=>"",
 "unit_amenities"=>["air conditioning"],
 "description"=>"",
 "rooms"=>"1"},
 "commit"=>"Publish Your Listing"}

エラー

NoMethodError in ApartmentsController#create
undefined method `each' for "1":String

考え？