diff --git a/ChangeLog b/ChangeLog
index d46431606e..826bc44aad 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Aug 13 17:31:12 2008  Yukihiro Matsumoto  <matz@ruby-lang.org>
+
+	* vm_insnhelper.c (caller_setup_args): should ensure if the value
+	  from to_proc is a real Proc.
+
 Wed Aug 13 17:28:29 2008  Akinori MUSHA  <knu@iDaemons.org>
 
 	* common.mk: regint.h no longer includes vm_core.h.
diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index 0fe9902418..a5967f736c 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -221,7 +221,7 @@ caller_setup_args(const rb_thread_t *th, rb_control_frame_t *cfp, VALUE flag,
 	    if (proc != Qnil) {
 		if (!rb_obj_is_proc(proc)) {
 		    VALUE b = rb_check_convert_type(proc, T_DATA, "Proc", "to_proc");
-		    if (NIL_P(b)) {
+		    if (NIL_P(b) || !rb_obj_is_proc(b)) {
 			rb_raise(rb_eTypeError,
 				 "wrong argument type %s (expected Proc)",
 				 rb_obj_classname(proc));