diff --git a/ChangeLog b/ChangeLog
index 7acdf7fce7..ffe004bb4a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Feb  3 12:13:20 2016  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* string.c (str_new_frozen): if the given string is embeddedable
+	  but not embedded, embed a new copied string. [Bug #11946]
+
 Wed Feb  3 08:25:38 2016  boshan  <boshan@subsplash.com>
 
 	* ext/openssl/ossl_pkey.c (Init_ossl_pkey): [DOC] Fix typo
diff --git a/ext/-test-/string/cstr.c b/ext/-test-/string/cstr.c
index 897793d7ed..9bd33a344e 100644
--- a/ext/-test-/string/cstr.c
+++ b/ext/-test-/string/cstr.c
@@ -1,5 +1,4 @@
-#include "ruby.h"
-#include "ruby/encoding.h"
+#include "internal.h"
 
 static VALUE
 bug_str_cstr_term(VALUE str)
@@ -71,6 +70,44 @@ bug_str_s_cstr_term_char(VALUE self, VALUE str)
     return bug_str_cstr_term_char(str);
 }
 
+#define TERM_LEN(str) rb_enc_mbminlen(rb_enc_get(str))
+#define TERM_FILL(ptr, termlen) do {\
+    char *const term_fill_ptr = (ptr);\
+    const int term_fill_len = (termlen);\
+    *term_fill_ptr = '\0';\
+    if (UNLIKELY(term_fill_len > 1))\
+	memset(term_fill_ptr, 0, term_fill_len);\
+} while (0)
+
+static VALUE
+bug_str_s_cstr_noembed(VALUE self, VALUE str)
+{
+    VALUE str2 = rb_str_new(NULL, 0);
+    long capacity = RSTRING_LEN(str) + TERM_LEN(str);
+    char *buf = ALLOC_N(char, capacity);
+    Check_Type(str, T_STRING);
+    FL_SET((str2), STR_NOEMBED);
+    memcpy(buf, RSTRING_PTR(str), capacity);
+    RBASIC(str2)->flags &= ~RSTRING_EMBED_LEN_MASK;
+    RSTRING(str2)->as.heap.aux.capa = capacity;
+    RSTRING(str2)->as.heap.ptr = buf;
+    RSTRING(str2)->as.heap.len = RSTRING_LEN(str);
+    TERM_FILL(RSTRING_END(str2), TERM_LEN(str));
+    return str2;
+}
+
+static VALUE
+bug_str_s_cstr_embedded_p(VALUE self, VALUE str)
+{
+    return STR_EMBED_P(str) ? Qtrue : Qfalse;
+}
+
+static VALUE
+bug_str_s_rb_str_new_frozen(VALUE self, VALUE str)
+{
+    return rb_str_new_frozen(str);
+}
+
 void
 Init_cstr(VALUE klass)
 {
@@ -80,4 +117,7 @@ Init_cstr(VALUE klass)
     rb_define_singleton_method(klass, "cstr_term", bug_str_s_cstr_term, 1);
     rb_define_singleton_method(klass, "cstr_unterm", bug_str_s_cstr_unterm, 2);
     rb_define_singleton_method(klass, "cstr_term_char", bug_str_s_cstr_term_char, 1);
+    rb_define_singleton_method(klass, "cstr_noembed", bug_str_s_cstr_noembed, 1);
+    rb_define_singleton_method(klass, "cstr_embedded?", bug_str_s_cstr_embedded_p, 1);
+    rb_define_singleton_method(klass, "rb_str_new_frozen", bug_str_s_rb_str_new_frozen, 1);
 }
diff --git a/string.c b/string.c
index 7af14c8ef2..b6de5f2b1b 100644
--- a/string.c
+++ b/string.c
@@ -1090,6 +1090,13 @@ str_new_frozen(VALUE klass, VALUE orig)
 		return shared;
 	    }
 	}
+	else if (RSTRING_LEN(orig)+TERM_LEN(orig) <= RSTRING_EMBED_LEN_MAX+1) {
+	    str = str_alloc(klass);
+	    STR_SET_EMBED(str);
+	    memcpy(RSTRING_PTR(str), RSTRING_PTR(orig), RSTRING_LEN(orig));
+	    STR_SET_EMBED_LEN(str, RSTRING_LEN(orig));
+	    TERM_FILL(RSTRING_END(str), TERM_LEN(orig));
+	}
 	else {
 	    str = str_alloc(klass);
 	    STR_SET_NOEMBED(str);
diff --git a/test/-ext-/string/test_cstr.rb b/test/-ext-/string/test_cstr.rb
index f4ed520b2d..35b5fe5931 100644
--- a/test/-ext-/string/test_cstr.rb
+++ b/test/-ext-/string/test_cstr.rb
@@ -36,6 +36,12 @@ class Test_StringCStr < Test::Unit::TestCase
     end
   end
 
+  def test_rb_str_new_frozen_embed
+    str = Bug::String.cstr_noembed("rbconfig.rb")
+    str = Bug::String.rb_str_new_frozen(str)
+    assert_equal true, Bug::String.cstr_embedded?(str)
+  end
+
   WCHARS = [Encoding::UTF_16BE, Encoding::UTF_16LE, Encoding::UTF_32BE, Encoding::UTF_32LE]
 
   def test_wchar_embed