diff --git a/ChangeLog b/ChangeLog
index 0546c1c690..008905a89f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Tue Jan  8 07:56:11 2008  Tanaka Akira  <akr@fsij.org>
+
+	* string.c (rb_str_buf_append): fix append itself.
+
 Tue Jan  8 01:13:50 2008  Tanaka Akira  <akr@fsij.org>
 
 	* string.c (STR_ENC_GET): defined.  same as rb_enc_get without
diff --git a/string.c b/string.c
index 95c4e0c440..189cc1b751 100644
--- a/string.c
+++ b/string.c
@@ -1166,7 +1166,13 @@ rb_enc_str_buf_cat(VALUE str, const char *ptr, long len, rb_encoding *ptr_enc)
 VALUE
 rb_str_buf_append(VALUE str, VALUE str2)
 {
-    int str2_cr = ENC_CODERANGE(str2);
+    int str2_cr;
+
+    if (str == str2) {
+        str2 = rb_str_dup(str2);
+    }
+
+    str2_cr = ENC_CODERANGE(str2);
 
     rb_enc_cr_str_buf_cat(str, RSTRING_PTR(str2), RSTRING_LEN(str2),
         ENCODING_GET(str2), str2_cr, &str2_cr);
diff --git a/test/ruby/test_string.rb b/test/ruby/test_string.rb
index 524ab47051..647c520707 100644
--- a/test/ruby/test_string.rb
+++ b/test/ruby/test_string.rb
@@ -170,6 +170,12 @@ class TestString < Test::Unit::TestCase
   def test_LSHIFT # '<<'
     assert_equal(S("world!"), S("world") << 33)
     assert_equal(S("world!"), S("world") << S('!'))
+
+    s = "a"
+    10.times {|i|
+      s << s
+      assert_equal("a" * (2<<i), s)
+    }
   end
 
   def test_MATCH # '=~'