diff --git a/ACU/Grading.pm b/ACU/Grading.pm
index af1f112..4c4fa22 100644
--- a/ACU/Grading.pm
+++ b/ACU/Grading.pm
@@ -342,12 +342,15 @@ sub compute ($$$;$$$)
 	$ref =~ s/\$LOGIN/$login/;
     }
 
-    my $glob = Tinyglob::tinyglob($ref);
-    if ($glob ne $ref)
+    if (defined $ref)
     {
-	my $value = 0;
-	$value += $ids->{$_} while (grep { /^$glob$/ } keys %$ids);
-	$ids->{ $ref } = $value;
+	my $glob = Tinyglob::tinyglob($ref);
+	if ($glob ne $ref)
+	{
+	    my $value = 0;
+	    $value += $ids->{$_} while (grep { /^$glob$/ } keys %$ids);
+	    $ids->{ $ref } = $value;
+	}
     }
 
     my $ret = undef;