diff --git a/Shorewall/Perl/Shorewall/Config.pm b/Shorewall/Perl/Shorewall/Config.pm
index 4c8cab1..350c2c3 100644
--- a/Shorewall/Perl/Shorewall/Config.pm
+++ b/Shorewall/Perl/Shorewall/Config.pm
@@ -2160,7 +2160,7 @@ sub evaluate_expression( $$$ ) {
 	#                         $1      $2   $3                     -     $4
 	while ( $expression =~ m( ^(.*?) \@({)? (\d+|[a-zA-Z]\w*) (?(2)}) (.*)$ )x ) {
 	    my ( $first, $var, $rest ) = ( $1, $3, $4);
-	    $var = numeric_value( $var ) if $var;
+	    $var = numeric_value( $var ) if $var =~ /^\d/;
 	    $val = $var ? $actparms{$var} : $chain;
 	    $parmsmodified ||= $var eq 'caller';
 	    $expression = join_parts( $first, $val, $rest );