diff --git a/Shorewall/Perl/Shorewall/Providers.pm b/Shorewall/Perl/Shorewall/Providers.pm
index f7fccc3fa..0db05c993 100644
--- a/Shorewall/Perl/Shorewall/Providers.pm
+++ b/Shorewall/Perl/Shorewall/Providers.pm
@@ -601,7 +601,7 @@ sub add_an_rtrule( ) {
 
     if ( $optional ) {
 	my $base = uc chain_base( $providers{$provider}{physical} );
-	emit ( '', "if [ -n \$${base}_IS_USABLE ]; then" );
+	emit ( '', qq(if [ -n "\$${base}_IS_USABLE" ]; then) );
 	push_indent;
     }