diff --git a/ChangeLog b/ChangeLog
index 03a8349834..3392441791 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Tue Oct  7 16:43:51 2008  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* ext/json/lib/json/pure/generator.rb (utf8_to_json):
+	  force_encoding before gsub.
+
 Tue Oct  7 16:35:41 2008  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* ext/json/lib/json/pure/generator.rb (utf8_to_json):
diff --git a/ext/json/lib/json/pure/generator.rb b/ext/json/lib/json/pure/generator.rb
index b44e42ac41..abc24bf72d 100644
--- a/ext/json/lib/json/pure/generator.rb
+++ b/ext/json/lib/json/pure/generator.rb
@@ -40,8 +40,8 @@ module JSON
   # Convert a UTF8 encoded Ruby string _string_ to a JSON string, encoded with
   # UTF16 big endian characters as \u????, and return it.
   def utf8_to_json(string) # :nodoc:
-    string = string.gsub(/["\\\/\x0-\x1f]/) { MAP[$&] }
-    string.force_encoding(Encoding::ASCII_8BIT)
+    string = string.dup.force_encoding(Encoding::ASCII_8BIT)
+    string.gsub!(/["\\\/\x0-\x1f]/) { MAP[$&] }
     string.gsub!(/(
                     (?:
                       [\xc2-\xdf][\x80-\xbf]    |