diff --git a/CmdLine.hs b/CmdLine.hs
index a4ef3ba..c93d69a 100644
--- a/CmdLine.hs
+++ b/CmdLine.hs
@@ -41,14 +41,20 @@ usage = do
 		]
 	exitFailure
 
-defaultMain :: (HostName -> [Property]) -> IO ()
+defaultMain :: (HostName -> Maybe [Property]) -> IO ()
 defaultMain getprops = go =<< processCmdLine
   where
-	go (Run host) = ensureProperties (getprops host)
+	go (Run host) = maybe (unknownhost host) ensureProperties (getprops host)
 	go (Spin host) = spin host
-	go (Boot host) = boot (getprops host)
+	go (Boot host) = maybe (unknownhost host) boot (getprops host)
 	go (Set host field) = setPrivData host field
 
+unknownhost :: HostName -> IO a
+unknownhost h = error $ unwords
+	[ "Unknown host:", h
+	, "(perhaps you should specify the real hostname on the command line?)"
+	]
+
 spin :: HostName -> IO ()
 spin host = do
 	url <- getUrl
diff --git a/Propellor.hs b/Propellor.hs
index 7a9e145..58d8289 100644
--- a/Propellor.hs
+++ b/Propellor.hs
@@ -19,13 +19,13 @@ main = defaultMain getProperties
 {- This is where the system's HostName, either as returned by uname
  - or one specified on the command line, is converted into a list of
  - Properties for that system. -}
-getProperties :: HostName -> [Property]
-getProperties hostname@"clam.kitenet.net" =
+getProperties :: HostName -> Maybe [Property]
+getProperties hostname@"clam.kitenet.net" = Just
 	[ cleanCloudAtCost hostname
 	, standardSystem Apt.Unstable
+	, Network.ipv6to4
 	-- Clam is a tor bridge, and an olduse.net shellbox.
 	, Tor.isBridge
-	, Network.ipv6to4
 	, JoeySites.oldUseNetshellBox
 	-- I play with docker on clam.
 	, Docker.configured
@@ -37,10 +37,7 @@ getProperties hostname@"clam.kitenet.net" =
 	]
 -- add more hosts here...
 --getProperties "foo" =
-getProperties h = error $ unwords
-	[ "Unknown host:", h
-	, "(perhaps you should specify the real hostname on the command line?)"
-	]
+getProperties _ = Nothing
 
 -- This is my standard system setup
 standardSystem :: Apt.Suite -> Property