diff --git a/ChangeLog b/ChangeLog
index 59f8e78d8b..5c50f32789 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Tue Jul 22 12:56:24 2014  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* string.c (rb_str_count): fix wrong single-byte optimization.
+	  7bit ascii can be a trailing byte in Shift_JIS.
+	  [ruby-dev:48442] [Bug #10078]
+
 Tue Jul 22 01:48:38 2014  Eric Wong  <e@80x24.org>
 
 	* include/ruby/io.h (rb_io_buffer_t): fix packing on gcc
diff --git a/string.c b/string.c
index 4feb7cfc2e..b303b56d89 100644
--- a/string.c
+++ b/string.c
@@ -6075,13 +6075,15 @@ rb_str_count(int argc, VALUE *argv, VALUE str)
     rb_check_arity(argc, 1, UNLIMITED_ARGUMENTS);
     for (i=0; i<argc; i++) {
 	VALUE tstr = argv[i];
-	unsigned char c;
+	const unsigned char *utstr;
 
 	StringValue(tstr);
 	enc = rb_enc_check(str, tstr);
 	if (argc == 1 && RSTRING_LEN(tstr) == 1 && rb_enc_asciicompat(enc) &&
-	    (c = RSTRING_PTR(tstr)[0]) < 0x80 && !is_broken_string(str)) {
+	    (utstr = (const OnigUChar *)RSTRING_PTR(tstr), ONIGENC_IS_ALLOWED_REVERSE_MATCH(enc, utstr, utstr+1)) &&
+	    !is_broken_string(str)) {
 	    int n = 0;
+	    unsigned char c = utstr[0];
 
 	    s = RSTRING_PTR(str);
 	    if (!s || RSTRING_LEN(str) == 0) return INT2FIX(0);
diff --git a/test/ruby/test_m17n.rb b/test/ruby/test_m17n.rb
index b9b85b2129..7d00b1aec6 100644
--- a/test/ruby/test_m17n.rb
+++ b/test/ruby/test_m17n.rb
@@ -1037,6 +1037,11 @@ class TestM17N < Test::Unit::TestCase
     assert_raise(Encoding::CompatibilityError){s.count(a("\xa3\xb0"))}
   end
 
+  def test_count_sjis_trailing_byte
+    bug10078 = '[ruby-dev:48442] [Bug #10078]'
+    assert_equal(0, s("\x98\x61").count("a"), bug10078)
+  end
+
   def test_delete
     assert_equal(1, e("\xa1\xa2").delete("z").length)
     s = e("\xa3\xb0\xa3\xb1\xa3\xb2\xa3\xb3\xa3\xb4")