diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Changelog
 
+## [0.0.1.4] - 2024-08-22
+
+### Changed
+
+* More accurate support for `EitherCodec` in `ObjectCodec`s.
+
 ## [0.0.1.3] - 2024-08-22
 
 ### Changed
diff --git a/autodocodec-nix.cabal b/autodocodec-nix.cabal
--- a/autodocodec-nix.cabal
+++ b/autodocodec-nix.cabal
@@ -5,7 +5,7 @@
 -- see: https://github.com/sol/hpack
 
 name:           autodocodec-nix
-version:        0.0.1.3
+version:        0.0.1.4
 synopsis:       Autodocodec interpreters for nix
 homepage:       https://github.com/NorfairKing/autodocodec#readme
 bug-reports:    https://github.com/NorfairKing/autodocodec/issues
diff --git a/src/Autodocodec/Nix.hs b/src/Autodocodec/Nix.hs
--- a/src/Autodocodec/Nix.hs
+++ b/src/Autodocodec/Nix.hs
@@ -106,10 +106,11 @@
 -- If Nix options ever figure out how to do optional fields, we'll use that
 -- instead.
 objectCodecNixOptions :: ObjectCodec input output -> Map Text Option
-objectCodecNixOptions = simplifyOptions . go
+objectCodecNixOptions = simplifyOptions . go False
   where
-    go :: ObjectCodec input output -> Map Text Option
-    go = \case
+    -- The bool means 'force optional'
+    go :: Bool -> ObjectCodec input output -> Map Text Option
+    go b = \case
       DiscriminatedUnionCodec k _ m ->
         M.insert
           k
@@ -127,14 +128,22 @@
                     optionDefault = Nothing
                   }
             )
-          $ map (go . snd)
+          $ map (go b . snd)
           $ HM.elems m
       RequiredKeyCodec key o mDesc ->
         M.singleton key $
           Option
-            { optionType = valueCodecNixOptionType o,
+            { optionType =
+                ( if b
+                    then fmap OptionTypeNullOr
+                    else id
+                )
+                  $ valueCodecNixOptionType o,
               optionDescription = mDesc,
-              optionDefault = Nothing -- [ref:NixOptionNullable]
+              optionDefault =
+                if b
+                  then Just JSON.Null
+                  else Nothing -- [ref:NixOptionNullable]
             }
       OptionalKeyCodec key o mDesc ->
         M.singleton key $
@@ -160,9 +169,9 @@
               optionDefault = Just $ toJSONVia c defaultValue
             }
       PureCodec _ -> M.empty
-      ApCodec c1 c2 -> M.union (go c1) (go c2)
-      BimapCodec _ _ c -> go c
-      EitherCodec _ c1 c2 -> M.union (go c1) (go c2) -- TODO use an or?
+      ApCodec c1 c2 -> M.union (go b c1) (go b c2)
+      BimapCodec _ _ c -> go b c
+      EitherCodec _ c1 c2 -> M.union (go True c1) (go True c2) -- TODO use a more accurate or?
 
 data Option = Option
   { optionType :: !(Maybe OptionType),