diff --git a/lib/set.rb b/lib/set.rb
index a8f4345..784d11c 100644
--- a/lib/set.rb
+++ b/lib/set.rb
@@ -87,7 +87,7 @@ class Set
     if block
       do_with_enum(enum) { |o| add(block[o]) }
     else
-      merge(enum)
+      merge!(enum)
     end
   end
 
@@ -155,7 +155,7 @@ class Set
     else
       do_with_enum(enum)  # make sure enum is enumerable before calling clear
       clear
-      merge(enum)
+      merge!(enum)
     end
   end
 
@@ -379,9 +379,13 @@ class Set
     self if size != n
   end
 
+  def merge(enum)
+    dup.merge!(enum)
+  end
+
   # Merges the elements of the given enumerable object to the set and
   # returns self.
-  def merge(enum)
+  def merge!(enum)
     if enum.instance_of?(self.class)
       @hash.update(enum.instance_variable_get(:@hash))
     else
@@ -401,7 +405,7 @@ class Set
   # Returns a new set built by merging the set and the elements of the
   # given enumerable object.
   def |(enum)
-    dup.merge(enum)
+    dup.merge!(enum)
   end
   alias + |             ##
   alias union |         ##
diff --git a/test/test_set.rb b/test/test_set.rb
index 66944f5..061d2d1 100644
--- a/test/test_set.rb
+++ b/test/test_set.rb
@@ -479,6 +479,14 @@ class TC_Set < Test::Unit::TestCase
     set = Set[1,2,3]
 
     ret = set.merge([2,4,6])
+    assert_not_same(set, ret)
+    assert_equal(Set[1,2,3,4,6], ret)
+  end
+
+ def test_merge!
+    set = Set[1,2,3]
+
+    ret = set.merge!([2,4,6])
     assert_same(set, ret)
     assert_equal(Set[1,2,3,4,6], set)
   end