diff --git a/Shorewall/Perl/Shorewall/Config.pm b/Shorewall/Perl/Shorewall/Config.pm
index ac8e595..57bb365 100644
--- a/Shorewall/Perl/Shorewall/Config.pm
+++ b/Shorewall/Perl/Shorewall/Config.pm
@@ -3025,9 +3025,13 @@ sub update_config_file( $ ) {
     my $annotate = shift;
 
     sub is_set( $ ) {
-	my $value = $config{$_[0]};
+	my $option = $_[0];
+
+	if ( defined $option ) {
+	    my $value = $config{$_[0]};
 	
-	defined( $value ) && lc( $value ) eq 'yes';
+	    defined( $value ) && lc( $value ) eq 'yes';
+	}
     }
 
     my $wide = is_set $config{WIDE_TC_MARKS};