Index: thread.c
===================================================================
--- thread.c	(リビジョン 31967)
+++ thread.c	(作業コピー)
@@ -1509,7 +1509,7 @@
 static VALUE
 rb_thread_s_kill(VALUE obj, VALUE th)
 {
-    if (CLASS_OF(th) != rb_cThread) {
+    if (!rb_obj_is_kind_of(th, rb_cThread)) {
         rb_raise(rb_eTypeError, 
                 "wrong argument type %s (expected Thread)",
                 rb_obj_classname(th));