diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Changelog
 
+## [0.0.1.0] - 2021-11-19
+
+### Added
+
+* `disjointEitherCodec` now does not generate `additionalProperties = true`.
+
 ## [0.0.0.0] - 2021-11-19
 
 First release.
diff --git a/autodocodec-swagger2.cabal b/autodocodec-swagger2.cabal
--- a/autodocodec-swagger2.cabal
+++ b/autodocodec-swagger2.cabal
@@ -1,11 +1,11 @@
 cabal-version: 1.12
 
--- This file has been generated from package.yaml by hpack version 0.34.4.
+-- This file has been generated from package.yaml by hpack version 0.34.5.
 --
 -- see: https://github.com/sol/hpack
 
 name:           autodocodec-swagger2
-version:        0.0.0.0
+version:        0.0.1.0
 synopsis:       Autodocodec interpreters for swagger2
 homepage:       https://github.com/NorfairKing/autodocodec#readme
 bug-reports:    https://github.com/NorfairKing/autodocodec/issues
@@ -34,7 +34,7 @@
       src
   build-depends:
       aeson
-    , autodocodec
+    , autodocodec >=0.0.1.0
     , base >=4.7 && <5
     , insert-ordered-containers
     , scientific
diff --git a/src/Autodocodec/Swagger/Schema.hs b/src/Autodocodec/Swagger/Schema.hs
--- a/src/Autodocodec/Swagger/Schema.hs
+++ b/src/Autodocodec/Swagger/Schema.hs
@@ -104,12 +104,12 @@
               { _schemaParamSchema = mempty {_paramSchemaEnum = Just [toJSONVia valCodec val]}
               }
       BimapCodec _ _ c -> go c
-      EitherCodec c1 c2 -> do
+      EitherCodec u c1 c2 -> do
         ns1 <- go c1
         let s1 = _namedSchemaSchema ns1
         ns2 <- go c2
         let s2 = _namedSchemaSchema ns2
-        pure $ NamedSchema Nothing $ combineSchemaOr s1 s2
+        pure $ NamedSchema Nothing $ combineSchemaOr u s1 s2
       CommentCodec t c -> do
         NamedSchema mName s <- go c
         pure $ NamedSchema mName $ addDoc t s
@@ -160,10 +160,10 @@
           ]
       OptionalKeyWithOmittedDefaultCodec key vs defaultValue mDoc -> goObject (OptionalKeyWithDefaultCodec key vs defaultValue mDoc)
       PureCodec _ -> pure []
-      EitherCodec oc1 oc2 -> do
+      EitherCodec u oc1 oc2 -> do
         ss1 <- goObject oc1
         ss2 <- goObject oc2
-        pure [combineSchemaOr (combineObjectSchemas ss1) (combineObjectSchemas ss2)]
+        pure [combineSchemaOr u (combineObjectSchemas ss1) (combineObjectSchemas ss2)]
       ApCodec oc1 oc2 -> do
         ss1 <- goObject oc1
         ss2 <- goObject oc2
@@ -180,8 +180,8 @@
         }
     combineObjectSchemas :: [Schema] -> Schema
     combineObjectSchemas = mconcat
-    combineSchemaOr :: Schema -> Schema -> Schema
-    combineSchemaOr s1 s2 =
+    combineSchemaOr :: Union -> Schema -> Schema -> Schema
+    combineSchemaOr u s1 s2 =
       let ps1 = _schemaParamSchema s1
           ps2 = _schemaParamSchema s2
           -- Swagger 2 doesn't support sum types so we have to work around that here.
@@ -195,7 +195,9 @@
           -- the schema: one which lets any value through.
           overApproximation =
             mempty
-              { _schemaAdditionalProperties = Just $ AdditionalPropertiesAllowed True
+              { _schemaAdditionalProperties = case u of
+                  PossiblyJointUnion -> Just $ AdditionalPropertiesAllowed True
+                  DisjointUnion -> Nothing
               }
        in case (,) <$> _paramSchemaEnum ps1 <*> _paramSchemaEnum ps2 of
             (Just (es1, es2)) ->