diff --git a/cameo/zstack.pl b/cameo/zstack.pl
index bd5d27f..5700a94 100755
--- a/cameo/zstack.pl
+++ b/cameo/zstack.pl
@@ -15,6 +15,7 @@
 sub usage
 {
 	print STDERR "usage: $0 [-r] z-start z-maxstep [file ...]\n";
+	print STDERR "  -r  reverse Z stacking. Also swaps X and Y.\n";
 	exit(1);
 }
 
@@ -39,6 +40,7 @@ if ($ARGV[0] eq "-r") {
 	shift @ARGV;
 	$reverse = 1;
 }
+&usage if $ARGV[0] =~ /^-[^0-9]/;
 
 $z0 = shift @ARGV;
 $zs = shift @ARGV;