diff --git a/Control/Shell.hs b/Control/Shell.hs
--- a/Control/Shell.hs
+++ b/Control/Shell.hs
@@ -95,7 +95,7 @@
   v <- lookupEnv key
   setEnv key $ f (maybe "" id v)
   x <- act
-  maybe (return ()) (setEnv key) v
+  setEnv key $ maybe "" id v
   return x
 
 -- | Get the value of an environment variable. Returns the empty string if
diff --git a/Control/Shell/Internal.hs b/Control/Shell/Internal.hs
--- a/Control/Shell/Internal.hs
+++ b/Control/Shell/Internal.hs
@@ -16,7 +16,7 @@
 import Control.Monad.IO.Class
 import qualified Control.Concurrent as Conc
 import qualified Control.Exception as Ex
-import qualified Data.Map as M
+import Data.List (sort)
 import qualified System.Directory as Dir
 import qualified System.Environment as Env
 import qualified System.Exit as Exit
@@ -71,12 +71,35 @@
 instance Functor Shell where
   fmap f (Shell x) = Shell (fmap (fmap (fmap f)) x)
 
+-- | Given a list of old key-value pairs and a list of new key-value pairs,
+--   returns the list of key-value pairs where the key exists in both the old
+--   and the new list, and the values are different between the new and the old
+--   list. The values in the returned list are taken from the old list.
+--   Keys must be unique.
+--   @oldValues [(a, 1), (c, 2)] [(b, 3), (c, 4)] == [(c, 2)]@
+oldValues :: (Ord a, Eq a, Eq b) => [(a, b)] -> [(a, b)] -> [(a, b)]
+oldValues xxs@((k1, v1):xs) yys@((k2, v2):ys)
+  | k1 <  k2             = oldValues xs yys
+  | k1 >  k2             = oldValues xxs ys
+  | k1 == k2 && v1 /= v2 = (k1, v1) : oldValues xs ys
+  | otherwise            = oldValues xs ys
+oldValues _ _            = []
+
+-- | Remove all given keys from the given key-value list. Both lists must be
+--   sorted, and the keys in the key-value list must be unique.
+(\\) :: (Ord a, Eq a) => [(a, b)] -> [(a, b)] -> [(a, b)]
+xxs@(x@(k1,_):xs) \\ kks@((k,_):ks)
+  | k <  k1 = xxs \\ ks
+  | k >  k1 = x:(xs \\ kks)
+  | k == k1 = xs \\ ks
+xs \\ _     = xs
+
 -- | Run a Shell computation. The program's working directory and environment
 --   will be restored after after the computation finishes.
 shell :: Shell a -> IO (Either ExitReason a)
 shell act = do
     dir <- Dir.getCurrentDirectory
-    env <- M.fromList <$> Env.getEnvironment
+    env <- sort <$> Env.getEnvironment
     (pids, res) <- unSh act
     merr <- waitPids pids
     Dir.setCurrentDirectory dir
@@ -88,12 +111,12 @@
     resultToEither (Next x) = Right x
     resultToEither (Fail e) = Left (Failure e)
     resultToEither (Done)   = Left Success
-    
+
+    resetEnv :: [(String, String)] -> IO ()
     resetEnv old = do
-      new <- M.fromList <$> Env.getEnvironment
-      mapM_ (Env.unsetEnv . fst) (M.toList (new M.\\ old))
-      mapM_ (uncurry Env.setEnv) (M.toList (M.filterWithKey (changed old) new))
-    changed old k v = maybe True (/= v) (M.lookup k old)
+      new <- sort <$> Env.getEnvironment
+      mapM_ (Env.unsetEnv . fst) (new \\ old)
+      mapM_ (uncurry Env.setEnv) (oldValues old new)
 
 -- | Run a shell computation and discard its return value. If the computation
 --   fails, print its error message to @stderr@ and exit.
diff --git a/shellmate.cabal b/shellmate.cabal
--- a/shellmate.cabal
+++ b/shellmate.cabal
@@ -1,5 +1,5 @@
 name:                shellmate
-version:             0.2
+version:             0.2.1
 synopsis:            Simple interface for shell scripting in Haskell.
 description:         Aims to simplify development of cross-platform shell scripts and similar things.
 homepage:            http://github.com/valderman/shellmate
@@ -26,17 +26,16 @@
     Control.Shell.Handle
     Control.Shell.Internal
   build-depends:
-    base >=4.7 && <5,
-    containers,
-    transformers >=0.3,
-    download-curl,
-    feed,
-    tagsoup,
-    xml,
-    bytestring,
-    filepath,
-    process,
-    directory,
-    temporary
+    base          >=4.7  && <5,
+    transformers  >=0.3  && <0.5,
+    download-curl >=0.1  && <0.2,
+    feed          >=0.3  && <0.4,
+    tagsoup       >=0.13 && <0.14,
+    xml           >=1.3  && <1.4,
+    bytestring    >=0.10 && <0.11,
+    filepath      >=1.3  && <1.5,
+    process       >=1.1  && <1.5,
+    directory     >=1.1  && <1.3,
+    temporary     >=1.1  && <1.3
   ghc-options:
     -Wall