diff --git a/Utility/OsPath.hs b/Utility/OsPath.hs
index bd7baf85eb..9fa4e0f57e 100644
--- a/Utility/OsPath.hs
+++ b/Utility/OsPath.hs
@@ -24,6 +24,8 @@ import "os-string" System.OsString.Internal.Types
 import qualified Data.ByteString.Short as S
 #if defined(mingw32_HOST_OS)
 import GHC.IO (unsafePerformIO)
+import System.OsString.Encoding.Internal (cWcharsToChars_UCS2)
+import qualified System.OsString.Data.ByteString.Short.Word16 as BS16
 #endif
 
 toOsPath :: RawFilePath -> OsPath
@@ -38,7 +40,10 @@ toOsPath = OsString . PosixString . S.toShort
 
 fromOsPath :: OsPath -> RawFilePath
 #if defined(mingw32_HOST_OS)
-fromOsPath = S.fromShort . getWindowsString . getOsString
+-- On Windows, OsString contains a ShortByteString that is
+-- utf-16 encoded. So have to convert the input from that.
+-- This is relatively expensive.
+fromOsPath = toRawFilePath . cWcharsToChars_UCS2 . BS16.unpack . getWindowsString
 #else
 fromOsPath = S.fromShort . getPosixString . getOsString
 #endif