diff --git a/ChangeLog b/ChangeLog
index 2c0ca81caa..84e813d4ec 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Mon Dec 24 05:13:04 2007  Yukihiro Matsumoto  <matz@ruby-lang.org>
+
+	* string.c (tr_trans): should associate new encoding if modified.
+
 Mon Dec 24 04:04:12 2007  GOTOU Yuuzou  <gotoyuzo@notwork.org>
 
 	* test/net/http/test_https.rb: should rescue LoadError.
diff --git a/string.c b/string.c
index 271960d730..60eadbd1db 100644
--- a/string.c
+++ b/string.c
@@ -3654,8 +3654,11 @@ tr_trans(VALUE str, VALUE src, VALUE repl, int sflag)
 	STR_SET_NOEMBED(str);
 	RSTRING(str)->as.heap.aux.capa = max;
     }
-
-    if (modify) return str;
+    
+    if (modify) {
+	rb_enc_associate(str, enc);
+	return str;
+    }
     return Qnil;
 }
 
diff --git a/test/ruby/test_m17n.rb b/test/ruby/test_m17n.rb
index 60c7c88d18..0026816335 100644
--- a/test/ruby/test_m17n.rb
+++ b/test/ruby/test_m17n.rb
@@ -1763,11 +1763,7 @@ class TestM17N < Test::Unit::TestCase
         next
       end
       t = s1.tr(s2, s3)
-      if s3.empty?
-        assert_equal(0, t.length, desc)
-        next
-      end
-      assert_equal(s1.length, t.length, desc)
+      assert_operator(s1.length, :>=, t.length, desc)
     }
   end
 
@@ -1799,12 +1795,7 @@ class TestM17N < Test::Unit::TestCase
       end
 
       t = nil
-      assert_nothing_raised(desc) { t = s1.tr_s(s2, s3) }
-
-      if s3.empty?
-        assert_equal(0, t.length, desc)
-        next
-      end
+      assert_nothing_raised(desc) { t = s1.tr_s(s1, s3) }
       assert_operator(s1.length, :>=, t.length, desc)
     }
   end