diff --git a/Cabal/src/Distribution/Simple/Flag.hs b/Cabal/src/Distribution/Simple/Flag.hs
index 671892aa96b..018a7288274 100644
--- a/Cabal/src/Distribution/Simple/Flag.hs
+++ b/Cabal/src/Distribution/Simple/Flag.hs
@@ -26,7 +26,6 @@ module Distribution.Simple.Flag (
   flagToMaybe,
   flagToList,
   maybeToFlag,
-  maybeStringToBoolFlag,
   BooleanFlag(..) ) where
 
 import Prelude ()
@@ -125,12 +124,6 @@ maybeToFlag :: Maybe a -> Flag a
 maybeToFlag Nothing  = NoFlag
 maybeToFlag (Just x) = Flag x
 
-maybeStringToBoolFlag :: Maybe String -> Flag Bool
-maybeStringToBoolFlag Nothing = NoFlag
-maybeStringToBoolFlag (Just "True") = Flag True
-maybeStringToBoolFlag (Just "False") = Flag False
-maybeStringToBoolFlag _ = NoFlag
-
 -- | Types that represent boolean flags.
 class BooleanFlag a where
     asBool :: a -> Bool
diff --git a/cabal-install/src/Distribution/Client/Setup.hs b/cabal-install/src/Distribution/Client/Setup.hs
index ef73a17aebf..b4653f977b6 100644
--- a/cabal-install/src/Distribution/Client/Setup.hs
+++ b/cabal-install/src/Distribution/Client/Setup.hs
@@ -88,7 +88,7 @@ import Distribution.Simple.Configure
 import qualified Distribution.Simple.Setup as Cabal
 import Distribution.Simple.Flag
          ( Flag(..), toFlag, flagToMaybe, flagToList, maybeToFlag
-         , flagElim, fromFlagOrDefault, maybeStringToBoolFlag
+         , flagElim, fromFlagOrDefault
          )
 import Distribution.Simple.Setup
          ( ConfigFlags(..), BuildFlags(..), ReplFlags
@@ -364,7 +364,7 @@ globalCommand commands = CommandUI {
       ,multiOption "nix"
         globalNix (\v flags -> flags { globalNix = v })
         [
-          optArg' "(True or False)" maybeStringToBoolFlag (\case
+          optArg' "(True or False)" (maybeToFlag . (readMaybe =<<)) (\case
             Flag True -> [Just "enable"]
             Flag False -> [Just "disable"]
             NoFlag -> [Just "disable"]) "" ["nix"]