diff --git a/usrp/fscan b/usrp/fscan
index 0e3b712..0dcbb4e 100755
--- a/usrp/fscan
+++ b/usrp/fscan
@@ -9,6 +9,7 @@ usage()
 
 
 [ "$1" ] || usage
+[ "${1#-}" = "$1" ] || usage
 [ "$3" ] && usage
 mkdir "$1" || exit
 pwr=${2:-2.6}