diff --git a/ChangeLog b/ChangeLog
index 5a67a9a1c7..6168208313 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Thu Jan 22 12:19:29 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* gc.c (define_final): cannot define finalizer for immediate
+	  values.  [ruby-core:21500]
+
+	* gc.c (define_final): freezes or hides internal values.
+
 Thu Jan 22 11:33:08 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* thread.c (rb_time_timeval): made a real prototype.  a patch from
diff --git a/gc.c b/gc.c
index ab882c50de..784949cf9e 100644
--- a/gc.c
+++ b/gc.c
@@ -2290,9 +2290,14 @@ define_final(int argc, VALUE *argv, VALUE os)
 	rb_raise(rb_eArgError, "wrong type argument %s (should be callable)",
 		 rb_obj_classname(block));
     }
-    FL_SET(obj, FL_FINALIZE);
+    if (!FL_ABLE(obj)) {
+	rb_raise(rb_eArgError, "cannot define finalizer for %s",
+		 rb_obj_classname(obj));
+    }
+    RBASIC(obj)->flags |= FL_FINALIZE;
 
     block = rb_ary_new3(2, INT2FIX(rb_safe_level()), block);
+    OBJ_FREEZE(block);
 
     if (!finalizer_table) {
 	finalizer_table = st_init_numtable();
@@ -2301,7 +2306,9 @@ define_final(int argc, VALUE *argv, VALUE os)
 	rb_ary_push(table, block);
     }
     else {
-	st_add_direct(finalizer_table, obj, rb_ary_new3(1, block));
+	table = rb_ary_new3(1, block);
+	RBASIC(table)->klass = 0;
+	st_add_direct(finalizer_table, obj, table);
     }
     return block;
 }