diff --git a/ChangeLog b/ChangeLog
index 058489a49b..c141f593d4 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Fri Jan 29 09:43:30 2010  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* enc/trans/utf8_mac.trans (buf_shift_char): don't see uninitialised
+	  value. [ruby-dev:40233]
+
 Fri Jan 29 01:42:24 2010  Yukihiro Matsumoto  <matz@ruby-lang.org>
 
 	* lib/delegate.rb (Delegator#initialize_copy): use initialize_copy
diff --git a/enc/trans/utf8_mac.trans b/enc/trans/utf8_mac.trans
index 7569c34ac0..ecda465355 100644
--- a/enc/trans/utf8_mac.trans
+++ b/enc/trans/utf8_mac.trans
@@ -85,10 +85,10 @@ buf_shift(struct from_utf8_mac_status *sp)
 void
 buf_shift_char(struct from_utf8_mac_status *sp)
 {
-    while (sp->beg != sp->end) {
+    if (sp->beg == sp->end) return;
+    do {
         buf_shift(sp);
-        if ((sp->buf[sp->beg] & 0xC0) != 0x80) break;
-    }
+    } while (sp->beg != sp->end && (sp->buf[sp->beg] & 0xC0) == 0x80);
 }
 
 void