--- loncom/homework/grades.pm	2010/02/05 14:52:28	1.592
+++ loncom/homework/grades.pm	2010/02/08 01:09:36	1.593
@@ -1,7 +1,7 @@
 # The LearningOnline Network with CAPA
 # The LON-CAPA Grading handler
 #
-# $Id: grades.pm,v 1.592 2010/02/05 14:52:28 bisitz Exp $
+# $Id: grades.pm,v 1.593 2010/02/08 01:09:36 raeburn Exp $
 #
 # Copyright Michigan State University Board of Trustees
 #
@@ -160,6 +160,10 @@ sub response_type {
         return;
     }
     my $res = $navmap->getBySymb($symb);
+    unless (ref($res)) {
+        $$response_error = 1;
+        return;
+    }
     my $partlist = $res->parts();
     my %vPart = 
 	map { $_ => 1 } (&Apache::loncommon::get_env_multiple('form.vPart'));