diff --git a/activesupport/lib/active_support/core_ext/enumerable.rb b/activesupport/lib/active_support/core_ext/enumerable.rb
index 434a32b29bba94b8e35e0594f32068fda6fcff29..e89b7e392e5456fa6b203e1512aac07656a08de7 100644
--- a/activesupport/lib/active_support/core_ext/enumerable.rb
+++ b/activesupport/lib/active_support/core_ext/enumerable.rb
@@ -55,12 +55,10 @@ def group_by
   #  [].sum(Payment.new(0)) { |i| i.amount } # => Payment.new(0)
   #
   def sum(identity = 0, &block)
-    return identity unless size > 0
-
     if block_given?
-      map(&block).sum
+      map(&block).sum(identity)
     else
-      inject { |sum, element| sum + element }
+      inject { |sum, element| sum + element } || identity
     end
   end
 
diff --git a/activesupport/test/core_ext/enumerable_test.rb b/activesupport/test/core_ext/enumerable_test.rb
index 885393815b9b2691e60f2cbc6d5c101863c0726b..1927a0ad0d5511a4c61b21789d4984fa4e020ffe 100644
--- a/activesupport/test/core_ext/enumerable_test.rb
+++ b/activesupport/test/core_ext/enumerable_test.rb
@@ -60,6 +60,10 @@ def test_empty_sums
     assert_equal Payment.new(0), [].sum(Payment.new(0))
   end
 
+  def test_enumerable_sums
+    assert_equal 10, (1..4).sum
+  end
+
   def test_each_with_object
     result = %w(foo bar).each_with_object({}) { |str, hsh| hsh[str] = str.upcase }
     assert_equal({'foo' => 'FOO', 'bar' => 'BAR'}, result)