diff --git a/Network/HTTP/Client/Manager.hs b/Network/HTTP/Client/Manager.hs
--- a/Network/HTTP/Client/Manager.hs
+++ b/Network/HTTP/Client/Manager.hs
@@ -324,9 +324,13 @@
 getConn :: Request
         -> Manager
         -> IO (ConnRelease, Connection, ManagedConn)
-getConn req m =
-    getManagedConn m (ConnKey connKeyHost connport (secure req)) $
-        go connaddr connhost connport
+getConn req m
+    -- Stop Mac OS X from getting high:
+    -- https://github.com/snoyberg/http-client/issues/40#issuecomment-39117909
+    | S8.null h = throwIO $ InvalidDestinationHost h
+    | otherwise =
+        getManagedConn m (ConnKey connKeyHost connport (secure req)) $
+            go connaddr connhost connport
   where
     h = host req
     (useProxy, connhost, connport) = getConnDest req
diff --git a/Network/HTTP/Client/Types.hs b/Network/HTTP/Client/Types.hs
--- a/Network/HTTP/Client/Types.hs
+++ b/Network/HTTP/Client/Types.hs
@@ -103,6 +103,7 @@
                    --
                    -- Since 1.9.4
                    | IncompleteHeaders
+                   | InvalidDestinationHost S.ByteString
     deriving (Show, T.Typeable)
 instance Exception HttpException
 
diff --git a/http-client.cabal b/http-client.cabal
--- a/http-client.cabal
+++ b/http-client.cabal
@@ -1,5 +1,5 @@
 name:                http-client
-version:             0.2.2.4
+version:             0.2.3
 synopsis:            An HTTP client engine, intended as a base layer for more user-friendly packages.
 description:         This codebase has been refactored from http-conduit.
 homepage:            https://github.com/snoyberg/http-client
diff --git a/test/Network/HTTP/ClientSpec.hs b/test/Network/HTTP/ClientSpec.hs
--- a/test/Network/HTTP/ClientSpec.hs
+++ b/test/Network/HTTP/ClientSpec.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE OverloadedStrings #-}
 module Network.HTTP.ClientSpec where
 
 import           Network.HTTP.Client
@@ -14,3 +15,13 @@
         man <- newManager defaultManagerSettings
         res <- httpLbs req man
         responseStatus res `shouldBe` status200
+    describe "fails on empty hostnames #40" $ do
+        let test url = it url $ do
+                req <- parseUrl url
+                man <- newManager defaultManagerSettings
+                _ <- httpLbs req man `shouldThrow` \e ->
+                    case e of
+                        InvalidDestinationHost "" -> True
+                        _ -> False
+                return ()
+        mapM_ test ["http://", "https://", "http://:8000", "https://:8001"]