diff --git a/ChangeLog b/ChangeLog
index 7e6378dc02..9ea0061680 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Thu Dec 11 01:21:58 2008  Yukihiro Matsumoto  <matz@ruby-lang.org>
+
+	* thread.c (rb_mutex_unlock): shut up warning.  a patch from
+	  Kazuhiro NISHIYAMA in [ruby-dev:37345]. fix #846
+
 Wed Dec 10 23:58:56 2008  Tadayoshi Funaba  <tadf@dotrb.org>
 
 	* complex.c: avoided warnings on cpp. [ruby-dev:37344]
diff --git a/thread.c b/thread.c
index 4fbc14d86c..4ab0788534 100644
--- a/thread.c
+++ b/thread.c
@@ -2978,7 +2978,7 @@ rb_mutex_unlock(VALUE self)
     GetMutexPtr(self, mutex);
 
     err = mutex_unlock(mutex);
-    if (err) rb_raise(rb_eThreadError, err);
+    if (err) rb_raise(rb_eThreadError, "%s", err);
 
     return self;
 }