diff --git a/upy.py b/upy.py
index dfda5befe53ddbb9d11a5b148286e1e289dc0d7e..4a33a96ae5b585eda936e5226cf0b146d9352ce3 100644
--- a/upy.py
+++ b/upy.py
@@ -21,6 +21,7 @@ char_delay = 0.0001
 import sys,serial,time
 if (len(sys.argv) < 3):
    print("upy.py port command [arguments]")
+   sys.exit()
 s = serial.Serial(sys.argv[1])
 s.write(b'\r\n\x03\r\n') # ^c break
 s.write(b'\x01') # ^a enter raw mode