diff --git a/Shorewall/Perl/Shorewall/Config.pm b/Shorewall/Perl/Shorewall/Config.pm
index c32b4b5a3..9c19f68e6 100644
--- a/Shorewall/Perl/Shorewall/Config.pm
+++ b/Shorewall/Perl/Shorewall/Config.pm
@@ -2481,7 +2481,7 @@ sub expand_variables( \$ ) {
 	my $val;
 
 	if ( $var =~ /^\d+$/ ) {
-	    fatal_error "Undefined parameter (\$$var)" unless defined $actparms[$var];
+	    fatal_error "Undefined parameter (\$$var)" if ( ! defined $actparms[$var] ) || ( length( $var ) > 0 && $var =~ /^0/ );
 	    $val = $var ? $actparms[$var] : $actparms[0]->{name};
 	} elsif ( exists $params{$var} ) {
 	    $val = $params{$var};