diff --git a/Shorewall/Perl/Shorewall/ARP.pm b/Shorewall/Perl/Shorewall/ARP.pm
index 38c4edc92..6def4864f 100644
--- a/Shorewall/Perl/Shorewall/ARP.pm
+++ b/Shorewall/Perl/Shorewall/ARP.pm
@@ -164,6 +164,7 @@ sub process_arprule() {
 
     if ( $opcode ne '-' ) {
 	my $invert = ( $opcode =~ s/^!// ) ? '! ' : '';
+	warning_message q(arptables versions through 0.3.4 ignore '!' after '--opcode') if $invert && ! $arptablesjf;
 	fatal_error "Invalid ARP OPCODE ($opcode)" unless $opcode =~ /^\d$/ && $opcode;
 	$rule .= $arptablesjf ? " --arpop ${invert}$map[$opcode] " : "--opcode ${invert}$opcode ";
     }