diff --git a/ChangeLog b/ChangeLog
index 81b3548b73..49844d4d3f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,4 +1,7 @@
-Fri Jul  8 16:39:03 2011  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+Fri Jul  8 16:40:38 2011  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* ext/json/parser/parser.rl (convert_encoding): should not modify
+	  the argument.
 
 	* ext/json/parser/parser.rl (convert_encoding): no needs to use
 	  force_encoding.
diff --git a/ext/json/parser/parser.c b/ext/json/parser/parser.c
index 817597ede4..04bedf33a7 100644
--- a/ext/json/parser/parser.c
+++ b/ext/json/parser/parser.c
@@ -1551,6 +1551,7 @@ static VALUE convert_encoding(VALUE source)
             } else if (len >= 4 && ptr[1] == 0 && ptr[3] == 0) {
                 source = rb_funcall(source, i_encode, 2, CEncoding_UTF_8, CEncoding_UTF_16LE);
             } else {
+                source = rb_str_dup(source);
                 FORCE_UTF8(source);
             }
         } else {
@@ -1694,16 +1695,16 @@ static VALUE cParser_parse(VALUE self)
     GET_PARSER;
 
 
-#line 1698 "parser.c"
+#line 1699 "parser.c"
 	{
 	cs = JSON_start;
 	}
 
-#line 695 "parser.rl"
+#line 696 "parser.rl"
     p = json->source;
     pe = p + json->len;
 
-#line 1707 "parser.c"
+#line 1708 "parser.c"
 	{
 	if ( p == pe )
 		goto _test_eof;
@@ -1780,7 +1781,7 @@ st10:
 	if ( ++p == pe )
 		goto _test_eof10;
 case 10:
-#line 1784 "parser.c"
+#line 1785 "parser.c"
 	switch( (*p) ) {
 		case 13: goto st10;
 		case 32: goto st10;
@@ -1837,7 +1838,7 @@ case 9:
 	_out: {}
 	}
 
-#line 698 "parser.rl"
+#line 699 "parser.rl"
 
     if (cs >= JSON_first_final && p == pe) {
         return result;
diff --git a/ext/json/parser/parser.rl b/ext/json/parser/parser.rl
index abd056f18c..07319a8fd7 100644
--- a/ext/json/parser/parser.rl
+++ b/ext/json/parser/parser.rl
@@ -549,6 +549,7 @@ static VALUE convert_encoding(VALUE source)
             } else if (len >= 4 && ptr[1] == 0 && ptr[3] == 0) {
                 source = rb_funcall(source, i_encode, 2, CEncoding_UTF_8, CEncoding_UTF_16LE);
             } else {
+                source = rb_str_dup(source);
                 FORCE_UTF8(source);
             }
         } else {
diff --git a/test/json/test_json.rb b/test/json/test_json.rb
index 4c724fd36e..5f186f0c08 100755
--- a/test/json/test_json.rb
+++ b/test/json/test_json.rb
@@ -398,4 +398,17 @@ EOT
     json = JSON::Parser.allocate
     assert_raises(TypeError, '[ruby-core:35079]') {json.source}
   end
+
+  def test_argument_encoding
+    source = "{}".force_encoding("ascii-8bit")
+    JSON::Parser.new(source)
+    assert_equal Encoding::ASCII_8BIT, source.encoding
+  end
+
+  def test_frozen_argument
+    source = "{}".force_encoding("ascii-8bit")
+    source.freeze
+    parser = nil
+    assert_nothing_raised {parser = JSON::Parser.new(source)}
+  end
 end