Allow nil to be used as a default_currency

lib/money/currency.rb

@@ -33,6 +33,9 @@ def initialize(method, currency, attribute)
     # Thrown when an unknown currency is requested.
     class UnknownCurrency < ArgumentError; end
 
+    # Thrown when currency is not provided.
+    class NoCurrency < ArgumentError; end
+
     class << self
       def new(id)
         id = id.to_s.downcase

lib/money/money.rb

@@ -156,7 +156,9 @@ def self.default_currency
       @using_deprecated_default_currency = false
     end
 
-    if @default_currency.respond_to?(:call)
+    if @default_currency.nil?
+      nil
+    elsif @default_currency.respond_to?(:call)
       Money::Currency.new(@default_currency.call)
     else
       Money::Currency.new(@default_currency)
@@ -338,7 +340,7 @@ class << self
   #   Money.new(100, "USD") #=> #<Money @fractional=100 @currency="USD">
   #   Money.new(100, "EUR") #=> #<Money @fractional=100 @currency="EUR">
   #
-  def initialize( obj, currency = Money.default_currency, options = {})
+  def initialize(obj, currency = Money.default_currency, options = {})
     # For backwards compatability, if options is not a Hash, treat it as a bank parameter
     unless options.is_a?(Hash)
       options = { bank: options }
@@ -352,6 +354,7 @@ def initialize( obj, currency = Money.default_currency, options = {})
 
     # BigDecimal can be Infinity and NaN, money of that amount does not make sense
     raise ArgumentError, 'must be initialized with a finite value' unless @fractional.finite?
+    raise Currency::NoCurrency, 'must be provide a currency' if @currency.nil?
   end
 
   # Assuming using a currency using dollars:

spec/money_spec.rb

@@ -70,6 +70,21 @@
       it "should have the default currency" do
         expect(money.currency).to eq Money.default_currency
       end
+
+      context 'without a default' do
+        around do |example|
+          default_currency = Money.default_currency
+          Money.default_currency = nil
+
+          example.run
+
+          Money.default_currency = default_currency
+        end
+
+        it 'should throw an NoCurrency Error' do
+          expect { money }.to raise_error(Money::Currency::NoCurrency)
+        end
+      end
     end
 
     context 'given a currency is provided' do