diff --git a/nav.py b/nav.py
index 4ed6603..71f9e2a 100644
--- a/nav.py
+++ b/nav.py
@@ -74,6 +74,8 @@ def print_plan(destination):
 
 
 def main():
+    if len(sys.argv) != 2:
+        sys.exit(__doc__)
     if sys.argv[1] == "--list":
         for key in sorted(ROUTES):
             print(f"{key:12} {NAMES[key]}")