diff --git a/winsup/cygwin/ChangeLog b/winsup/cygwin/ChangeLog
index 74ca18851..b887f4200 100644
--- a/winsup/cygwin/ChangeLog
+++ b/winsup/cygwin/ChangeLog
@@ -1,3 +1,8 @@
+2009-05-08  IWAMURO Motonori <deenheart@gmail.com>
+
+	* strfuncs.cc (sys_cp_wcstombs): Set errno to 0 before converting
+	wide char to SO/UTF-8 sequence.
+
 2009-05-08  Corinna Vinschen  <corinna@vinschen.de>
 
 	* include/sys/select.h: Guard definitions with __USE_W32_SOCKETS as
diff --git a/winsup/cygwin/strfuncs.cc b/winsup/cygwin/strfuncs.cc
index 110fe8f16..75ca3db28 100644
--- a/winsup/cygwin/strfuncs.cc
+++ b/winsup/cygwin/strfuncs.cc
@@ -432,6 +432,7 @@ sys_cp_wcstombs (wctomb_p f_wctomb, char *charset, char *dst, size_t len,
          ASCII SO; UTF-8 representation of invalid char. */
       if (bytes == -1 && *charset != 'U'/*TF-8*/)
         {
+	  _REENT->_errno = 0;
 	  buf[0] = 0x0e; /* ASCII SO */
 	  bytes = __utf8_wctomb (_REENT, buf + 1, pw, charset, &ps);
 	  if (bytes == -1)