diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,24 @@
 # Changelog
 
+## [0.7.0.0] - 2026-10-07
+
+### Fixed
+
+* `optionalFieldOrNullWithOmittedDefault`, `optionalFieldOrNullWithOmittedDefault'`,
+  `optionalFieldOrNullWithOmittedDefaultWith` and
+  `optionalFieldOrNullWithOmittedDefaultWith'` now omit the key when the value
+  is the default value, as they document, instead of encoding `null`.
+  Both encodings parse back to the default value, so this changes the output
+  rather than what can be read.
+
+* `unsafeUnboundedNaturalCodec` no longer throws an arithmetic underflow when
+  it is given a negative number.  It fails to parse instead.
+
+### Removed
+
+* The `Result` newtype and its instances.
+  Nothing used it: it was left over from the time codecs it was written for.
+
 ## [0.6.0.0] - 2026-07-14
 
 This is technically a breaking change but it's unlikely that you'll need to
diff --git a/Setup.hs b/Setup.hs
new file mode 100644
--- /dev/null
+++ b/Setup.hs
@@ -0,0 +1,2 @@
+import Distribution.Simple
+main = defaultMain
diff --git a/autodocodec.cabal b/autodocodec.cabal
--- a/autodocodec.cabal
+++ b/autodocodec.cabal
@@ -5,7 +5,7 @@
 -- see: https://github.com/sol/hpack
 
 name:           autodocodec
-version:        0.6.0.0
+version:        0.7.0.0
 synopsis:       Self-documenting encoder and decoder
 homepage:       https://github.com/NorfairKing/autodocodec#readme
 bug-reports:    https://github.com/NorfairKing/autodocodec/issues
@@ -37,6 +37,7 @@
       Paths_autodocodec
   hs-source-dirs:
       src
+  ghc-options: -Wall -Wincomplete-uni-patterns -Wincomplete-record-updates -Wpartial-fields -Widentities -Wredundant-constraints -Wcpp-undef
   build-depends:
       aeson
     , base >=4.7 && <5
@@ -61,7 +62,7 @@
       Paths_autodocodec
   hs-source-dirs:
       doctest/
-  ghc-options: -main-is DocTest -threaded -rtsopts -with-rtsopts=-N -Wall
+  ghc-options: -Wall -Wincomplete-uni-patterns -Wincomplete-record-updates -Wpartial-fields -Widentities -Wredundant-constraints -Wcpp-undef -main-is DocTest -threaded -rtsopts -with-rtsopts=-N -Wall
   build-depends:
       base >=4.7 && <5
     , doctest
diff --git a/src/Autodocodec/Codec.hs b/src/Autodocodec/Codec.hs
--- a/src/Autodocodec/Codec.hs
+++ b/src/Autodocodec/Codec.hs
@@ -6,7 +6,6 @@
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE GADTs #-}
-{-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE MultiWayIf #-}
 {-# LANGUAGE RankNTypes #-}
@@ -1178,7 +1177,11 @@
     f = \case
       Just v -> v
       Nothing -> defaultValue
-    g v = if v == defaultValue then Nothing else Just v
+    -- Not 'if v == defaultValue then Nothing else Just v': the underlying
+    -- codec omits the key when what it is given equals its own default, which
+    -- is 'Just defaultValue', so answering 'Nothing' for the default value is
+    -- what would write @null@ instead of omitting the key.
+    g = Just
 
 -- | Like 'optionalFieldWithOmittedDefaultWith'', but the value may also be
 -- @null@ and that will be interpreted as the default value.
@@ -1193,10 +1196,11 @@
   ObjectCodec output output
 optionalFieldOrNullWithOmittedDefaultWith' key c defaultValue = dimapCodec f g $ optionalFieldWithOmittedDefaultWith' key (maybeCodec c) (Just defaultValue)
   where
+    -- See the note on 'optionalFieldOrNullWithOmittedDefaultWith'.
     f = \case
       Just v -> v
       Nothing -> defaultValue
-    g v = if v == defaultValue then Nothing else Just v
+    g = Just
 
 -- | An optional, or null, field
 --
@@ -1630,8 +1634,14 @@
 unsafeUnboundedNaturalCodec =
   bimapCodec go fromIntegral scientificCodec
   where
-    go s = case Scientific.floatingOrInteger s :: Either Float Natural of
-      Right i -> Right i
+    -- Via 'Integer' rather than straight to 'Natural': 'floatingOrInteger'
+    -- reaches its result type through 'fromInteger', and 'fromInteger' into
+    -- 'Natural' throws on a negative number, so asking it for a 'Natural'
+    -- turns a number this codec should refuse into an arithmetic underflow.
+    go s = case Scientific.floatingOrInteger s :: Either Float Integer of
+      Right i
+        | i < 0 -> Left ("Number is negative: " <> show s)
+        | otherwise -> Right (fromInteger i)
       Left _ -> Left ("Number is not an integer: " <> show s)
 
 -- | A codec for a literal piece of 'Text'.
@@ -2078,17 +2088,6 @@
   Text ->
   JSONCodec a
 codecViaAeson doc = bimapCodec (JSON.parseEither JSON.parseJSON) JSON.toJSON valueCodec <?> doc
-
--- Could get this from https://hackage.haskell.org/package/either-result-0.3.1.0/docs/Control-Monad-Result.html#t:Result
--- but just reimplementing here to avoid a dependency, as it's not exported anyway
--- (well it is actually, until we give this module an explicit export list).
--- We need to do this because `Either String a` doesn't have a `MonadFail` instance,
--- but `Time.iso8601ParseM` expects it's return value to have a `MonadFail` instance.
-newtype Result a = Result {runResult :: Either String a}
-  deriving newtype (Functor, Applicative, Monad)
-
-instance MonadFail Result where
-  fail = Result . Left
 
 -- TODO 'aeson' has it's own custom datetime serialising code in the module @Data.Aeson.Encoding.Builder@:
 -- The core function here is `Data.Aeson.Encoding.Builder.timeOfDay64`.