diff --git a/res/ndn b/res/ndn
index 3cdef2e..99bdc07 100755
--- a/res/ndn
+++ b/res/ndn
@@ -1,7 +1,7 @@
 #!/bin/sh
 
-if [[ $# -eq 0 ]] ; then
-    echo ERROR: No command specified
+if [[ $# -eq 0 ]] || [[ ! -f /Applications/NDN.app/Contents/Platform/"$@" ]] ; then
+    echo ERROR: No command or invalid command is specified
     echo
     echo Available commands:
     for command in $(ls /Applications/NDN.app/Contents/Platform/ 2>/dev/null); do