added error check

svn: r1454
2005-12-01 04:09:33 +00:00 · 2005-12-01 04:09:33 +00:00 · 3d570f8d32
commit 3d570f8d32
parent 23347d028a
1 changed files with 1 additions and 0 deletions
--- 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)