diff --git a/ChangeLog b/ChangeLog
index 38950333c1..1bb9d1f085 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Fri May  1 16:50:05 2009  Akinori MUSHA  <knu@iDaemons.org>
+
+	* lib/set.rb (Set#merge): Only directly use the passed objects
+	  @hash instance variable when self and the passed object are
+	  instances of the same class. [Bug #118]
+
 Fri May  1 16:44:11 2009  Akinori MUSHA  <knu@iDaemons.org>
 
 	* lib/set.rb (SortedSet#add): Do not let an uncomparable object
diff --git a/lib/set.rb b/lib/set.rb
index ec42b93fab..5be2bce76a 100644
--- a/lib/set.rb
+++ b/lib/set.rb
@@ -276,8 +276,8 @@ class Set
   # Merges the elements of the given enumerable object to the set and
   # returns self.
   def merge(enum)
-    if enum.is_a?(Set)
-      @hash.update(enum.instance_eval { @hash })
+    if enum.instance_of?(self.class)
+      @hash.update(enum.instance_variable_get(:@hash))
     else
       enum.each { |o| add(o) }
     end