From 3d570f8d32506983f0e175725710f43b49a6096d Mon Sep 17 00:00:00 2001
From: Robby Findler <robby@racket-lang.org>
Date: Thu, 1 Dec 2005 04:09:33 +0000
Subject: [PATCH] added error check

svn: r1454
---
 collects/htdp/world.ss | 1 +
 1 file changed, 1 insertion(+)

diff --git a/collects/htdp/world.ss b/collects/htdp/world.ss
index 55807ebb1e..12c316e437 100644
--- a/collects/htdp/world.ss
+++ b/collects/htdp/world.ss
@@ -233,6 +233,7 @@
   
   (define (update-frame pict)
     (unless the-frame (error 'update SEQUENCE-ERROR))
+    (check-result 'update (lambda (x) (beg:image? x)) "image" pict)
     (send txt begin-edit-sequence)
     (send txt lock #f)
     (send txt delete 0 (send txt last-position) #f)