diff --git a/ChangeLog b/ChangeLog
index ff5ae43c38..da7c7eeee3 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sat Jun 13 00:45:08 2015  Marc-Andre Lafortune  <ruby-core@marc-andre.ca>
+
+	* lib/prime.rb: Return sized enumerators.
+	  Patch by Kenichi Kamiya [GH-931]
+
 Sat Jun 13 00:45:06 2015  Marc-Andre Lafortune  <ruby-core@marc-andre.ca>
 
 	* lib/prime.rb: Fix with_object with no block given
diff --git a/lib/prime.rb b/lib/prime.rb
index 1eed28d914..c9575f6a2a 100644
--- a/lib/prime.rb
+++ b/lib/prime.rb
@@ -270,7 +270,7 @@ class Prime
 
     # see +Enumerator+#with_index.
     def with_index(offset = 0)
-      return enum_for(:with_index, offset) unless block_given?
+      return enum_for(:with_index, offset) { Float::INFINITY } unless block_given?
       return each_with_index(&proc) if offset == 0
 
       each do |prime|
@@ -281,11 +281,15 @@ class Prime
 
     # see +Enumerator+#with_object.
     def with_object(obj)
-      return enum_for(:with_object, obj) unless block_given?
+      return enum_for(:with_object, obj) { Float::INFINITY } unless block_given?
       each do |prime|
         yield prime, obj
       end
     end
+
+    def size
+      Float::INFINITY
+    end
   end
 
   # An implementation of +PseudoPrimeGenerator+.
diff --git a/test/test_prime.rb b/test/test_prime.rb
index d4f056466f..48b017e2bc 100644
--- a/test/test_prime.rb
+++ b/test/test_prime.rb
@@ -106,6 +106,13 @@ class TestPrime < Test::Unit::TestCase
     end
   end
 
+  def test_enumerator_size
+    enum = Prime.each
+    assert_equal Float::INFINITY, enum.size
+    assert_equal Float::INFINITY, enum.with_object(nil).size
+    assert_equal Float::INFINITY, enum.with_index(42).size
+  end
+
   def test_default_instance_does_not_have_compatibility_methods
     assert !Prime.instance.respond_to?(:succ)
     assert !Prime.instance.respond_to?(:next)