diff --git a/ChangeLog b/ChangeLog
index 326e89763b..8b1c5a099e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Mon Oct  1 12:50:59 2007  Yukihiro Matsumoto  <matz@ruby-lang.org>
+
+	* gc.c (id2ref): valid id should not refer T_VALUE nor T_ICLASS.
+	  [ruby-dev:31911]
+
 Wed Sep 26 23:54:37 2007  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* ext/extmk.rb (extmake), lib/mkmf.rb (configuration): top_srcdir
diff --git a/gc.c b/gc.c
index 052df8f362..4298a8acf1 100644
--- a/gc.c
+++ b/gc.c
@@ -1922,6 +1922,7 @@ id2ref(obj, objid)
     VALUE obj, objid;
 {
     unsigned long ptr, p0;
+    int type;
 
     rb_secure(4);
     p0 = ptr = NUM2ULONG(objid);
@@ -1938,7 +1939,8 @@ id2ref(obj, objid)
         return ID2SYM(symid);
     }
 
-    if (!is_pointer_to_heap((void *)ptr)|| BUILTIN_TYPE(ptr) >= T_BLKTAG) {
+    if (!is_pointer_to_heap((void *)ptr)||
+	(type = BUILTIN_TYPE(ptr)) >= T_BLKTAG || type == T_ICLASS) {
 	rb_raise(rb_eRangeError, "0x%lx is not id value", p0);
     }
     if (BUILTIN_TYPE(ptr) == 0 || RBASIC(ptr)->klass == 0) {