diff --git a/ChangeLog b/ChangeLog
index aa0e7e124f..9da6568926 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Tue Feb  4 12:55:31 2014  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* string.c (rb_str_modify_expand): enable capacity and disable
+	  assocation with packed objects when setting capa, so that
+	  pack("p") string fails to unpack properly after modified.
+
 Tue Feb  4 12:45:15 2014  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* dir.c (glob_make_pattern): all alphabets are magic characters on
diff --git a/string.c b/string.c
index ee57f92716..983c2a1166 100644
--- a/string.c
+++ b/string.c
@@ -1503,6 +1503,7 @@ rb_str_modify_expand(VALUE str, long expand)
 	int termlen = TERM_LEN(str);
 	if (!STR_EMBED_P(str)) {
 	    REALLOC_N(RSTRING(str)->as.heap.ptr, char, capa + termlen);
+	    STR_UNSET_NOCAPA(str);
 	    RSTRING(str)->as.heap.aux.capa = capa;
 	}
 	else if (capa + termlen > RSTRING_EMBED_LEN_MAX + 1) {
diff --git a/test/ruby/test_pack.rb b/test/ruby/test_pack.rb
index 621a06beba..3f0931bdc0 100644
--- a/test/ruby/test_pack.rb
+++ b/test/ruby/test_pack.rb
@@ -181,6 +181,7 @@ class TestPack < Test::Unit::TestCase
     assert_equal a[0], a.pack("p").unpack("p")[0]
     assert_equal a, a.pack("p").freeze.unpack("p*")
     assert_raise(ArgumentError) { (a.pack("p") + "").unpack("p*") }
+    assert_raise(ArgumentError) { (a.pack("p") << "d").unpack("p*") }
   end
 
   def test_format_string_modified