--- /sys/lib/dist/pc/mkfile	Sun Aug  4 04:31:14 2013
+++ /sys/lib/dist/pc/mkfile	Tue Aug 20 20:25:53 2013
@@ -81,6 +81,10 @@
 	ls -l $target
 
 usbdisk:D: /n/sources/plan9
+	if (~ $#scr 0 || ~ $scr '') {
+		echo '$scr unset' >[1=2]
+		exit not-set-up
+	}
 	cd $scr
 	mkusbboot -s 1900 -r /n/sources/plan9 && mv image $target
 	rm -f $target.bz2