diff --git a/ChangeLog b/ChangeLog
index 3654044a06..395978ac80 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Fri Jan 11 01:08:01 2008  Yukihiro Matsumoto  <matz@ruby-lang.org>
+
+	* thread.c (rb_mutex_unlock): proper error message for unlocking
+	  mutex that is not locked.  a patch from Yusuke ENDOH
+	  <mame at tsg.ne.jp> in [ruby-dev:33010].
+
 Thu Jan 10 18:00:41 2008  Yukihiro Matsumoto  <matz@ruby-lang.org>
 
 	* prelude.rb (Mutex::synchronize): capture exception from unlock.
diff --git a/thread.c b/thread.c
index 31679bb9ed..e5b79b7330 100644
--- a/thread.c
+++ b/thread.c
@@ -2362,22 +2362,30 @@ VALUE
 rb_mutex_unlock(VALUE self)
 {
     mutex_t *mutex;
+    char *err = NULL;
     GetMutexPtr(self, mutex);
 
-    if (mutex->th != GET_THREAD()) {
-	rb_raise(rb_eThreadError,
-		 "Attempt to unlock a mutex which is locked by another thread");
+    native_mutex_lock(&mutex->lock);
+
+    if (mutex->th == 0) {
+	err = "Attempt to unlock a mutex which is not locked";
+    }
+    else if (mutex->th != GET_THREAD()) {
+	err = "Attempt to unlock a mutex which is locked by another thread";
+    }
+    else {
+	mutex->th = 0;
+	if (mutex->cond_waiting > 0) {
+	    /* waiting thread */
+	    native_cond_signal(&mutex->cond);
+	    mutex->cond_waiting--;
+	}
     }
 
-    native_mutex_lock(&mutex->lock);
-    mutex->th = 0;
-    if (mutex->cond_waiting > 0) {
-	/* waiting thread */
-	native_cond_signal(&mutex->cond);
-	mutex->cond_waiting--;
-    }
     native_mutex_unlock(&mutex->lock);
 
+    if (err) rb_raise(rb_eThreadError, err);
+
     return self;
 }
 
diff --git a/version.h b/version.h
index 698088098d..eeedb4ce53 100644
--- a/version.h
+++ b/version.h
@@ -1,7 +1,7 @@
 #define RUBY_VERSION "1.9.0"
-#define RUBY_RELEASE_DATE "2008-01-10"
+#define RUBY_RELEASE_DATE "2008-01-11"
 #define RUBY_VERSION_CODE 190
-#define RUBY_RELEASE_CODE 20080110
+#define RUBY_RELEASE_CODE 20080111
 #define RUBY_PATCHLEVEL 0
 
 #define RUBY_VERSION_MAJOR 1
@@ -9,7 +9,7 @@
 #define RUBY_VERSION_TEENY 0
 #define RUBY_RELEASE_YEAR 2008
 #define RUBY_RELEASE_MONTH 1
-#define RUBY_RELEASE_DAY 10
+#define RUBY_RELEASE_DAY 11
 
 #ifdef RUBY_EXTERN
 RUBY_EXTERN const char ruby_version[];