diff --git a/Shorewall/Perl/Shorewall/Tc.pm b/Shorewall/Perl/Shorewall/Tc.pm
index 486028f13..433d5add9 100644
--- a/Shorewall/Perl/Shorewall/Tc.pm
+++ b/Shorewall/Perl/Shorewall/Tc.pm
@@ -2262,7 +2262,7 @@ sub process_traffic_shaping() {
 		    my $rule = "run_tc class add dev $device parent $devicenumber:$parent classid $classid hfsc";
 
 		    if ( $dmax ) {
-			my $umax = $tcref->{umax} ? "$tcref->{umax}b" : "\${${dev}_mtu}b";
+			my $umax = $tcref->{umax} ? "$tcref->{umax}b" : "\$(get_device_mtu $device)b";
 			$rule .= " sc umax $umax dmax ${dmax}ms";
 			$rule .= " rate $rate" if $rawrate;
 		    } else {