diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,8 @@
+0.1.20220814
+------------
+
+Windows: Fix crash when `du` is not available.
+
 0.1.20210924
 ------------
 
diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -91,7 +91,7 @@
 --------
 
 List build artifacts of current project,
-marking superseded ones that can be deleded:
+marking superseded ones that can be deleted:
 
     cabal-clean
 
diff --git a/cabal-clean.cabal b/cabal-clean.cabal
--- a/cabal-clean.cabal
+++ b/cabal-clean.cabal
@@ -1,7 +1,7 @@
 cabal-version:       >=1.10
 
 name:                cabal-clean
-version:             0.1.20210924
+version:             0.1.20220814
 synopsis:            Remove outdated cabal build artefacts from `dist-newstyle`.
 
 description:         Simple command line tool to remove cabal build artefacts
@@ -30,18 +30,14 @@
   GHC == 8.6.5
   GHC == 8.8.4
   GHC == 8.10.7
-  GHC == 9.0.1
-  GHC == 9.2.0.20210821
+  GHC == 9.0.2
+  GHC == 9.2.4
+  GHC == 9.4.1
 
 source-repository head
   type:     git
   location: git://github.com/andreasabel/cabal-clean.git
 
-source-repository this
-  type:     git
-  location: git://github.com/andreasabel/cabal-clean.git
-  tag:      v0.1.20210924
-
 executable cabal-clean
   main-is:             Main.hs
 
@@ -49,6 +45,7 @@
     License
     Options
     Structure
+    Types
     Util
     Version
     Paths_cabal_clean
diff --git a/src/Main.hs b/src/Main.hs
--- a/src/Main.hs
+++ b/src/Main.hs
@@ -19,8 +19,6 @@
 
 import System.Console.Pretty
   ( supportsPretty )
-import System.IO
-  ( hPutStr, hPutStrLn, stderr )
 
 import License
   ( copyright, license )
@@ -178,15 +176,3 @@
       , "(E.g., there could be symlinks to executables stored there.)"
       ]
     ]
-
--- * Verbosity functionality.
-
-chat :: Options -> String -> IO ()
-chat = chatGen $ hPutStr stderr
-
-chatLn :: Options -> String -> IO ()
-chatLn = chatGen $ hPutStrLn stderr
-
-chatGen :: (String -> IO ()) -> Options -> String -> IO ()
-chatGen prt o msg = when (optVerbose o) $
-  prt $ styleOpt o Faint $ unwords ["info:", msg]
diff --git a/src/Options.hs b/src/Options.hs
--- a/src/Options.hs
+++ b/src/Options.hs
@@ -5,7 +5,7 @@
   ( Color(Green, Red, White)
   , Style(ColoredNormal, Faint, Italic)
   )
-import System.FilePath (FilePath)
+import Util
 
 data Options = Options
   { optDelete     :: Bool
@@ -26,3 +26,15 @@
 applyWhenColors opts
   | optNoColors opts = \ _ _ -> id
   | otherwise        = id
+
+-- * Verbosity functionality.
+
+chat :: Options -> String -> IO ()
+chat = chatGen $ hPutStr stderr
+
+chatLn :: Options -> String -> IO ()
+chatLn = chatGen $ hPutStrLn stderr
+
+chatGen :: (String -> IO ()) -> Options -> String -> IO ()
+chatGen prt o msg = when (optVerbose o) $
+  prt $ styleOpt o Faint $ unwords ["info:", msg]
diff --git a/src/Structure.hs b/src/Structure.hs
--- a/src/Structure.hs
+++ b/src/Structure.hs
@@ -39,6 +39,7 @@
 import Util
 
 import Options
+import Types
 
 -- | The structure of the build directory.
 
@@ -67,25 +68,6 @@
   , obsolete :: Bool
   } deriving Show
 
--- | We treat the architecture identifier as opaque.
-type Arch = String
-
--- | A package is given by its name.
-type Package = String
-
--- | A package version is a list of natural numbers.
-type PackageVersion  = NumericVersion
-
--- | A GHC major version is a list of natural numbers.
-type MajorVersion = NumericVersion
-
--- | A GHC minor version is a list of natural numbers.
-type MinorVersion = NumericVersion
-
-type CompilerVersion = (MajorVersion, MinorVersion)
-
-type NumericVersion = [Int]
-
 -- * Loading the build tree from disc.
 
 type Warnings = [String]
@@ -133,16 +115,19 @@
 -- or a new compiler minor version.
 
 markObsolete :: BuildTree -> BuildTree
-markObsolete (BuildTree t) = BuildTree $
-  flip fmap t $ modifyDesc $ \case
-    [] -> []
-    (ver, m) : vms ->
-      (ver, flip (fmap . fmap) m $ modifyDesc $ \case
-        [] -> []
-        me : mes -> me : map (second markEntryObsolete) mes
-      ) : map (second $ fmap $ fmap $ fmap markEntryObsolete) vms
-
+markObsolete =
+  -- for each Package, from highest to lowest PackageVersion:
+  modifyBuildTree $ fmap $ modifyDesc $ modifyCons
+    -- keep the highest PackageVersion, but iterate through GHC MinorVersion
+    (second $ fmap $ fmap $ modifyDesc $ modifyCons
+      -- keep the highest MinorVersion
+      id
+      -- mark lower MinorVersion as obsolete
+      (map $ second markEntryObsolete))
+    -- mark lower PackageVersion as obsolete
+    (map $ second $ fmap $ fmap $ fmap markEntryObsolete)
   where
+  modifyBuildTree f (BuildTree t) = BuildTree (f t)
   modifyDesc f = Map.fromDescList . f . Map.toDescList
   -- mapDesc f = Map.fromDescList . map (second f) . Map.toDescList
 
@@ -162,8 +147,7 @@
 
 printBuildTree :: Options -> BuildTree -> IO ()
 printBuildTree opts = foldMapEntry $ \ (Entry dir obsolete) -> do
-  (exitcode, stdout, _stderr) <- readProcessWithExitCode "du" ["-hs", dir] ""
-  let s = if exitcode == ExitSuccess then stdout else dir ++ "\n"
+  s <- readProcess "du" ["-hs", dir] "" `catchIOError` \ _ -> pure (dir ++ "\n")
   putStr $ colorize obsolete s
   where
   colorize True  = colorOpt opts Red   . ("---\t" ++)
@@ -205,32 +189,9 @@
 foldMapEntry :: Monoid m => (Entry -> m) -> BuildTree -> m
 foldMapEntry f (BuildTree t) = (foldMap . foldMap . foldMap . foldMap . foldMap) f t
 
--- * Parsing directory names
-
-parseKey :: Arch -> CompilerString -> PackageString -> Maybe Key
-parseKey arch hc s = do
-  (major, minor) <- parseCompilerString hc
-  (pkg  , ver  ) <- parsePackageString s
-  return $ Key pkg ver arch major minor
-
-type CompilerString = String
-
-parseCompilerString :: CompilerString -> Maybe CompilerVersion
-parseCompilerString s = do
-  n <- findIndex (== '-') s
-  case splitAt n s of
-    ("ghc", _:v) -> splitAt 2 <$> parseVersionString v
-    _ -> Nothing
-
-type PackageString = String
-
-parsePackageString :: PackageString -> Maybe (Package, PackageVersion)
-parsePackageString s = do
-  n <- findIndexEnd (== '-') s
-  let (p, _:v) = splitAt n s
-  (p,) <$> parseVersionString v
-
-type VersionString = String
-
-parseVersionString :: VersionString -> Maybe NumericVersion
-parseVersionString = mapM readMaybe . splitWhen (== '.')
+-- -- UNUSED
+-- parseKey :: Arch -> CompilerString -> PackageString -> Maybe Key
+-- parseKey arch hc s = do
+--   (major, minor) <- parseCompilerString hc
+--   (pkg  , ver  ) <- parsePackageString s
+--   return $ Key pkg ver arch major minor
diff --git a/src/Types.hs b/src/Types.hs
new file mode 100644
--- /dev/null
+++ b/src/Types.hs
@@ -0,0 +1,48 @@
+-- | Common data structures and types for the project.
+
+module Types where
+
+import Util
+
+-- | We treat the architecture identifier as opaque.
+type Arch = String
+
+-- | A package is given by its name.
+type Package = String
+
+-- | A package version is a list of natural numbers.
+type PackageVersion  = NumericVersion
+
+-- | A GHC major version is a list of natural numbers.
+type MajorVersion = NumericVersion
+
+-- | A GHC minor version is a list of natural numbers.
+type MinorVersion = NumericVersion
+
+type CompilerVersion = (MajorVersion, MinorVersion)
+
+type NumericVersion = [Int]
+
+-- * Parsing directory names
+
+type CompilerString = String
+
+parseCompilerString :: CompilerString -> Maybe CompilerVersion
+parseCompilerString s = do
+  n <- findIndex (== '-') s
+  case splitAt n s of
+    ("ghc", _:v) -> splitAt 2 <$> parseVersionString v
+    _ -> Nothing
+
+type PackageString = String
+
+parsePackageString :: PackageString -> Maybe (Package, PackageVersion)
+parsePackageString s = do
+  n <- findIndexEnd (== '-') s
+  let (p, _:v) = splitAt n s
+  (p,) <$> parseVersionString v
+
+type VersionString = String
+
+parseVersionString :: VersionString -> Maybe NumericVersion
+parseVersionString = mapM readMaybe . splitWhen (== '.')
diff --git a/src/Util.hs b/src/Util.hs
--- a/src/Util.hs
+++ b/src/Util.hs
@@ -20,7 +20,9 @@
 import System.Directory       as X (doesDirectoryExist, listDirectory, removeDirectoryRecursive)
 import System.Exit            as X (die, ExitCode(..))
 import System.FilePath        as X ((</>))
-import System.Process         as X (readProcessWithExitCode)
+import System.Process         as X (readProcess)
+import System.IO              as X (hPutStr, hPutStrLn, stderr)
+import System.IO.Error        as X (catchIOError)
 
 import Text.Read              as X (readMaybe)
 
@@ -43,3 +45,12 @@
 last1 :: a -> [a] -> a
 last1 a []     = a
 last1 _ (a:as) = last1 a as
+
+modifyCons :: (a -> a) -> ([a] -> [a]) -> [a] -> [a]
+modifyCons f g = \case
+  []   -> []
+  x:xs -> f x : g xs
+
+-- UNUSED
+modifyTail :: ([a] -> [a]) -> [a] -> [a]
+modifyTail = modifyCons id