diff --git a/nav.py b/nav.py
index ca03c84..eff3ebd 100644
--- a/nav.py
+++ b/nav.py
@@ -56,6 +56,9 @@ def plan(destination):
 
 def main():
     destination = sys.argv[1]
+    if destination not in ROUTES:
+        known = ", ".join(sorted(ROUTES))
+        sys.exit(f"Unknown destination: {destination} (known: {known})")
     route, total, days, fuel = plan(destination)
     print("Route: " + " -> ".join(NAMES[w] for w in route))
     print(f"Distance: {total:.1f} million km")