diff --git a/ChangeLog b/ChangeLog
index 38b3b0e029..abb0508be6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+Fri Jun 10 16:38:13 2011  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* encoding.c (rb_locale_charmap): When ruby process is run as Windows
+	  Service the console codepage is not set, GetConsoleCP returns 0.
+	  So on such environment, use GetACP().
+	  http://blogs.msdn.com/b/michkap/archive/2005/02/08/369197.aspx
+	  patched by Rafal Bigaj [ruby-core:36832] [Bug #4854]
+
 Fri Jun 10 14:34:24 2011  Koichi Sasada  <ko1@atdot.net>
 
 	* common.mk: restore TESTRUN_SCRIPT to "$(srcdir)/test.rb".
diff --git a/encoding.c b/encoding.c
index a88d176c0f..d152d77321 100644
--- a/encoding.c
+++ b/encoding.c
@@ -1427,7 +1427,9 @@ rb_locale_charmap(VALUE klass)
     const char *codeset = nl_langinfo_codeset();
     char cp[sizeof(int) * 3 + 4];
     if (!codeset) {
-	snprintf(cp, sizeof(cp), "CP%d", GetConsoleCP());
+	UINT codepage = GetConsoleCP();
+	if(!codepage) codepage = GetACP();
+	snprintf(cp, sizeof(cp), "CP%d", codepage);
 	codeset = cp;
     }
     return rb_usascii_str_new2(codeset);