diff --git a/.gitignore b/.gitignore
index 4a277591..c2cab955 100644
--- a/.gitignore
+++ b/.gitignore
@@ -17,3 +17,4 @@ cabal.sandbox.config
 *.aux
 *.hp
 .stack-work/
+stack.yaml.lock
diff --git a/src/Data/OpenApi.hs b/src/Data/OpenApi.hs
index a9ce8f3d..3b1e775f 100644
--- a/src/Data/OpenApi.hs
+++ b/src/Data/OpenApi.hs
@@ -30,7 +30,6 @@ module Data.OpenApi (
 
   -- * Re-exports
   module Data.OpenApi.Lens,
-  module Data.OpenApi.Optics,
   module Data.OpenApi.Operation,
   module Data.OpenApi.ParamSchema,
   module Data.OpenApi.Schema,
diff --git a/src/Data/OpenApi/Declare.hs b/src/Data/OpenApi/Declare.hs
index a302bb0a..6158bb9f 100644
--- a/src/Data/OpenApi/Declare.hs
+++ b/src/Data/OpenApi/Declare.hs
@@ -52,7 +52,7 @@ instance (Applicative m, Monad m, Monoid d) => Applicative (DeclareT d m) where
     return (mappend d' d'', f x)
 
 instance (Applicative m, Monad m, Monoid d) => Monad (DeclareT d m) where
-  return x = DeclareT (\_ -> pure (mempty, x))
+  return = pure
   DeclareT dx >>= f = DeclareT $ \d -> do
     ~(d',  x) <- dx d
     ~(d'', y) <- runDeclareT (f x) (mappend d d')
diff --git a/src/Data/OpenApi/Internal.hs b/src/Data/OpenApi/Internal.hs
index b9be5292..aea8a654 100644
--- a/src/Data/OpenApi/Internal.hs
+++ b/src/Data/OpenApi/Internal.hs
@@ -14,6 +14,8 @@
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE UndecidableInstances #-}
+{-# OPTIONS_GHC -fno-warn-orphans #-}
+
 module Data.OpenApi.Internal where
 
 import Prelude ()
@@ -335,7 +337,9 @@ instance Data MediaType where
 
   dataTypeOf _ = mediaTypeData
 
+mediaTypeConstr :: Constr
 mediaTypeConstr = mkConstr mediaTypeData "MediaType" [] Prefix
+mediaTypeData :: DataType
 mediaTypeData = mkDataType "MediaType" [mediaTypeConstr]
 
 instance Hashable MediaType where
@@ -1006,12 +1010,12 @@ deriveGeneric ''OpenApiSpecVersion
 -- =======================================================================
 
 instance Semigroup OpenApiSpecVersion where
-  (<>) (OpenApiSpecVersion a) (OpenApiSpecVersion b) = OpenApiSpecVersion $ max a b 
-  
+  (<>) (OpenApiSpecVersion a) (OpenApiSpecVersion b) = OpenApiSpecVersion $ max a b
+
 instance Monoid OpenApiSpecVersion where
   mempty = OpenApiSpecVersion (makeVersion [3,0,0])
   mappend = (<>)
-  
+
 instance Semigroup OpenApi where
   (<>) = genericMappend
 instance Monoid OpenApi where
@@ -1282,7 +1286,7 @@ instance FromJSON OAuth2AuthorizationCodeFlow where
 -- Manual ToJSON instances
 -- =======================================================================
 
-instance ToJSON OpenApiSpecVersion where 
+instance ToJSON OpenApiSpecVersion where
   toJSON (OpenApiSpecVersion v)= toJSON . showVersion $ v
 
 instance ToJSON MediaType where
@@ -1456,15 +1460,15 @@ instance FromJSON OpenApiSpecVersion where
   parseJSON = withText "OpenApiSpecVersion" $ \str ->
             let validatedVersion :: Either String Version
                 validatedVersion = do
-                  parsedVersion <- readVersion str 
+                  parsedVersion <- readVersion str
                   unless ((parsedVersion >= lowerOpenApiSpecVersion) && (parsedVersion <= upperOpenApiSpecVersion)) $
                      Left ("The provided version " <> showVersion parsedVersion <> " is out of the allowed range >=" <> showVersion lowerOpenApiSpecVersion <> " && <=" <> showVersion upperOpenApiSpecVersion)
                   return parsedVersion
-             in 
+             in
               either fail (return . OpenApiSpecVersion) validatedVersion
     where
     readVersion :: Text -> Either String Version
-    readVersion v = case readP_to_S parseVersion (Text.unpack v) of 
+    readVersion v = case readP_to_S parseVersion (Text.unpack v) of
       [] -> Left $ "Failed to parse as a version string " <> Text.unpack v
       solutions -> Right (fst . last $ solutions)
 
@@ -1649,7 +1653,7 @@ instance HasSwaggerAesonOptions Encoding where
 instance HasSwaggerAesonOptions Link where
   swaggerAesonOptions _ = mkSwaggerAesonOptions "link"
 
-instance AesonDefaultValue Version where 
+instance AesonDefaultValue Version where
   defaultValue = Just (makeVersion [3,0,0])
 instance AesonDefaultValue OpenApiSpecVersion
 instance AesonDefaultValue Server
diff --git a/src/Data/OpenApi/Internal/AesonUtils.hs b/src/Data/OpenApi/Internal/AesonUtils.hs
index 3804ab32..786226a1 100644
--- a/src/Data/OpenApi/Internal/AesonUtils.hs
+++ b/src/Data/OpenApi/Internal/AesonUtils.hs
@@ -4,6 +4,7 @@
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE ExplicitForAll #-}
 {-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE UndecidableSuperClasses #-}
 module Data.OpenApi.Internal.AesonUtils (
     -- * Generic functions
diff --git a/src/Data/OpenApi/Internal/ParamSchema.hs b/src/Data/OpenApi/Internal/ParamSchema.hs
index 75b637a2..a841b3ad 100644
--- a/src/Data/OpenApi/Internal/ParamSchema.hs
+++ b/src/Data/OpenApi/Internal/ParamSchema.hs
@@ -19,6 +19,7 @@ module Data.OpenApi.Internal.ParamSchema where
 
 import Control.Lens
 import Data.Aeson (ToJSON (..))
+import Data.Kind
 import Data.Proxy
 import GHC.Generics
 
@@ -163,7 +164,7 @@ instance ToParamSchema Word64 where
 --     "minimum": -128,
 --     "type": "integer"
 -- }
-toParamSchemaBoundedIntegral :: forall a t. (Bounded a, Integral a) => Proxy a -> Schema
+toParamSchemaBoundedIntegral :: forall a. (Bounded a, Integral a) => Proxy a -> Schema
 toParamSchemaBoundedIntegral _ = mempty
   & type_ ?~ OpenApiInteger
   & minimum_ ?~ fromInteger (toInteger (minBound :: a))
@@ -310,10 +311,10 @@ instance ToParamSchema UUID where
 --     ],
 --     "type": "string"
 -- }
-genericToParamSchema :: forall a t. (Generic a, GToParamSchema (Rep a)) => SchemaOptions -> Proxy a -> Schema
+genericToParamSchema :: forall a. (Generic a, GToParamSchema (Rep a)) => SchemaOptions -> Proxy a -> Schema
 genericToParamSchema opts _ = gtoParamSchema opts (Proxy :: Proxy (Rep a)) mempty
 
-class GToParamSchema (f :: * -> *) where
+class GToParamSchema (f :: Type -> Type) where
   gtoParamSchema :: SchemaOptions -> Proxy f -> Schema -> Schema
 
 instance GToParamSchema f => GToParamSchema (D1 d f) where
@@ -331,7 +332,7 @@ instance ToParamSchema c => GToParamSchema (K1 i c) where
 instance (GEnumParamSchema f, GEnumParamSchema g) => GToParamSchema (f :+: g) where
   gtoParamSchema opts _ = genumParamSchema opts (Proxy :: Proxy (f :+: g))
 
-class GEnumParamSchema (f :: * -> *) where
+class GEnumParamSchema (f :: Type -> Type) where
   genumParamSchema :: SchemaOptions -> Proxy f -> Schema -> Schema
 
 instance (GEnumParamSchema f, GEnumParamSchema g) => GEnumParamSchema (f :+: g) where
diff --git a/src/Data/OpenApi/Internal/Schema.hs b/src/Data/OpenApi/Internal/Schema.hs
index da56acf0..212b2a40 100644
--- a/src/Data/OpenApi/Internal/Schema.hs
+++ b/src/Data/OpenApi/Internal/Schema.hs
@@ -24,7 +24,7 @@ module Data.OpenApi.Internal.Schema where
 import Prelude ()
 import Prelude.Compat
 
-import Control.Lens hiding (allOf)
+import Control.Lens hiding (allOf, anyOf)
 import Data.Data.Lens (template)
 
 import Control.Applicative ((<|>))
@@ -43,6 +43,7 @@ import qualified Data.HashMap.Strict.InsOrd as InsOrdHashMap
 import Data.Int
 import Data.IntSet (IntSet)
 import Data.IntMap (IntMap)
+import Data.Kind
 import Data.List (sort)
 import Data.List.NonEmpty.Compat (NonEmpty)
 import Data.Map (Map)
@@ -587,7 +588,7 @@ sketchStrictSchema = go . toJSON
       where
         names = objectKeys o
 
-class GToSchema (f :: * -> *) where
+class GToSchema (f :: Type -> Type) where
   gdeclareNamedSchema :: SchemaOptions -> Proxy f -> Schema -> Declare (Definitions Schema) NamedSchema
 
 instance {-# OVERLAPPABLE #-} ToSchema a => ToSchema [a] where
@@ -623,7 +624,10 @@ instance ToSchema Float       where declareNamedSchema = plain . paramSchemaToSc
 instance (Typeable (Fixed a), HasResolution a) => ToSchema (Fixed a) where declareNamedSchema = plain . paramSchemaToSchema
 
 instance ToSchema a => ToSchema (Maybe a) where
-  declareNamedSchema _ = declareNamedSchema (Proxy :: Proxy a)
+  declareNamedSchema _ = do
+    ref <- declareSchemaRef (Proxy @a)
+    -- NB: using 'oneOf' goes wrong for nested Maybe's as both subschemas match 'null'.
+    pure $ unnamed $ mempty & anyOf ?~ [Inline $ mempty & type_ ?~ OpenApiNull, ref]
 
 instance (ToSchema a, ToSchema b) => ToSchema (Either a b) where
   -- To match Aeson instance
@@ -1016,10 +1020,7 @@ instance {-# OVERLAPPING #-} (Selector s, ToSchema c) => GToSchema (S1 s (K1 i (
 instance {-# OVERLAPPABLE #-} (Selector s, GToSchema f) => GToSchema (S1 s f) where
   gdeclareNamedSchema opts _ = fmap unnamed . withFieldSchema opts (Proxy2 :: Proxy2 s f) True
 
-instance {-# OVERLAPPING #-} ToSchema c => GToSchema (K1 i (Maybe c)) where
-  gdeclareNamedSchema _ _ _ = declareNamedSchema (Proxy :: Proxy c)
-
-instance {-# OVERLAPPABLE #-} ToSchema c => GToSchema (K1 i c) where
+instance ToSchema c => GToSchema (K1 i c) where
   gdeclareNamedSchema _ _ _ = declareNamedSchema (Proxy :: Proxy c)
 
 instance ( GSumToSchema f
@@ -1031,7 +1032,9 @@ instance ( GSumToSchema f
 
 gdeclareNamedSumSchema :: GSumToSchema f => SchemaOptions -> Proxy f -> Schema -> Declare (Definitions Schema) NamedSchema
 gdeclareNamedSumSchema opts proxy _
-  | allNullaryToStringTag opts && allNullary = pure $ unnamed (toStringTag sumSchemas)
+  | allNullaryToStringTag opts && allNullary = pure $ unnamed $ mempty
+      & type_ ?~ OpenApiString
+      & enum_ ?~ map (String . fst) sumSchemas
   | otherwise = do
     (schemas, _) <- runWriterT declareSumSchema
     return $ unnamed $ mempty
@@ -1040,13 +1043,9 @@ gdeclareNamedSumSchema opts proxy _
     declareSumSchema = gsumToSchema opts proxy
     (sumSchemas, All allNullary) = undeclare (runWriterT declareSumSchema)
 
-    toStringTag schemas = mempty
-      & type_ ?~ OpenApiString
-      & enum_ ?~ map (String . fst) sumSchemas
-
 type AllNullary = All
 
-class GSumToSchema (f :: * -> *)  where
+class GSumToSchema (f :: Type -> Type)  where
   gsumToSchema :: SchemaOptions -> Proxy f -> WriterT AllNullary (Declare (Definitions Schema)) [(T.Text, Referenced Schema)]
 
 instance (GSumToSchema f, GSumToSchema g) => GSumToSchema (f :+: g) where
diff --git a/src/Data/OpenApi/Internal/TypeShape.hs b/src/Data/OpenApi/Internal/TypeShape.hs
index 89230e21..cf3a1c6a 100644
--- a/src/Data/OpenApi/Internal/TypeShape.hs
+++ b/src/Data/OpenApi/Internal/TypeShape.hs
@@ -3,9 +3,11 @@
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE UndecidableInstances #-}
+{-# OPTIONS_GHC -Wno-unticked-promoted-constructors #-}
 
 module Data.OpenApi.Internal.TypeShape where
 
+import Data.Kind
 import Data.Proxy
 import GHC.Generics
 import GHC.TypeLits
@@ -46,7 +48,7 @@ type family GenericHasSimpleShape t (f :: Symbol) (s :: TypeShape) :: Constraint
       )
 
 -- | Infer a 'TypeShape' for a generic representation of a type.
-type family GenericShape (g :: * -> *) :: TypeShape
+type family GenericShape (g :: Type -> Type) :: TypeShape
 
 type instance GenericShape (f :*: g)        = ProdCombine  (GenericShape f) (GenericShape g)
 type instance GenericShape (f :+: g)        = SumCombine   (GenericShape f) (GenericShape g)
diff --git a/src/Data/OpenApi/Optics.hs b/src/Data/OpenApi/Optics.hs
index 3d0a42e8..588b50ee 100644
--- a/src/Data/OpenApi/Optics.hs
+++ b/src/Data/OpenApi/Optics.hs
@@ -4,6 +4,7 @@
 {-# LANGUAGE OverloadedLabels #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE UndecidableInstances #-}
 {-# OPTIONS_GHC -fno-warn-orphans #-}
 -- |
diff --git a/src/Data/OpenApi/Schema/Generator.hs b/src/Data/OpenApi/Schema/Generator.hs
index 9cb4014f..951d4345 100644
--- a/src/Data/OpenApi/Schema/Generator.hs
+++ b/src/Data/OpenApi/Schema/Generator.hs
@@ -34,6 +34,8 @@ schemaGen _ schema
     | Just cases <- schema  ^. enum_  = elements cases
 schemaGen defns schema
     | Just variants <- schema ^. oneOf = schemaGen defns =<< elements (dereference defns <$> variants)
+schemaGen defns schema
+    | Just variants <- schema ^. anyOf = schemaGen defns =<< elements (dereference defns <$> variants)
 schemaGen defns schema =
     case schema ^. type_ of
       Nothing ->
diff --git a/src/Data/OpenApi/Schema/Validation.hs b/src/Data/OpenApi/Schema/Validation.hs
index 9728ceef..fbbdecb8 100644
--- a/src/Data/OpenApi/Schema/Validation.hs
+++ b/src/Data/OpenApi/Schema/Validation.hs
@@ -34,10 +34,13 @@ import Data.OpenApi.Internal.Schema.Validation
 -- $setup
 -- >>> import Control.Lens
 -- >>> import Data.Aeson
+-- >>> import Data.Aeson.QQ.Simple
 -- >>> import Data.Proxy
 -- >>> import Data.OpenApi
+-- >>> import Data.OpenApi.Declare
 -- >>> import GHC.Generics
 -- >>> :set -XDeriveGeneric
+-- >>> :set -XQuasiQuotes
 
 -- $howto
 --
@@ -67,24 +70,18 @@ import Data.OpenApi.Internal.Schema.Validation
 
 -- $maybe
 --
--- Because @'Maybe' a@ has the same schema as @a@, validation
--- generally fails for @null@ JSON:
---
--- >>> validateToJSON (Nothing :: Maybe String)
--- ["expected JSON value of type OpenApiString"]
--- >>> validateToJSON ([Just "hello", Nothing] :: [Maybe String])
--- ["expected JSON value of type OpenApiString"]
--- >>> validateToJSON (123, Nothing :: Maybe String)
--- ["expected JSON value of type OpenApiString"]
---
--- However, when @'Maybe' a@ is a type of a record field,
--- validation takes @'required'@ property of the @'Schema'@
--- into account:
+-- The behavior is in line with "aeson" behavior for derived instances.
+-- When @'Maybe' a@ is a type of a record field,
+-- validation accepts both ommited field and null as a field value:
 --
 -- >>> data Person = Person { name :: String, age :: Maybe Int } deriving Generic
 -- >>> instance ToJSON Person
 -- >>> instance ToSchema Person
--- >>> validateToJSON (Person "Nick" (Just 24))
+-- >>> let (defs, sch) = runDeclare (declareSchema (Proxy :: Proxy Person)) mempty
+-- >>> let validate = validateJSON defs sch
+-- >>> validate [aesonQQ|{"name" : "Nick", "age" : 18}|]
+-- []
+-- >>> validate [aesonQQ|{"name" : "Nick", "age" : null}|]
 -- []
--- >>> validateToJSON (Person "Nick" Nothing)
+-- >>> validate [aesonQQ|{"name" : "Nick"}|]
 -- []
diff --git a/stack.yaml b/stack.yaml
index 7ab2f4e0..16179a8d 100644
--- a/stack.yaml
+++ b/stack.yaml
@@ -1,9 +1,5 @@
-resolver: lts-16.31
+resolver: lts-21.24
 packages:
 - '.'
-extra-deps:
-- optics-core-0.3
-- optics-th-0.3
-- optics-extra-0.3
-- indexed-profunctors-0.1
-- insert-ordered-containers-0.2.3.1
+ghc-options:
+  $locals: -Wall -Wno-unused-imports -Wno-dodgy-imports -Wno-name-shadowing
diff --git a/test/Data/OpenApi/CommonTestTypes.hs b/test/Data/OpenApi/CommonTestTypes.hs
index 99a39ec7..65480041 100644
--- a/test/Data/OpenApi/CommonTestTypes.hs
+++ b/test/Data/OpenApi/CommonTestTypes.hs
@@ -205,7 +205,14 @@ personSchemaJSON = [aesonQQ|
     {
       "name":   { "type": "string"  },
       "phone":  { "type": "integer" },
-      "email":  { "type": "string"  }
+      "email":
+        {
+          "anyOf" :
+            [
+              { "type" : "null" },
+              { "type": "string" }
+            ]
+        }
     },
   "required": ["name", "phone"]
 }
@@ -867,7 +874,14 @@ singleMaybeFieldSchemaJSON = [aesonQQ|
   "type": "object",
   "properties":
     {
-      "singleMaybeField": { "type": "string" }
+      "singleMaybeField":
+        {
+          "anyOf" :
+            [
+              { "type" : "null" },
+              { "type": "string" }
+            ]
+        }
     }
 }
 |]
diff --git a/test/Data/OpenApi/Schema/GeneratorSpec.hs b/test/Data/OpenApi/Schema/GeneratorSpec.hs
index 092673f6..7b3891cb 100644
--- a/test/Data/OpenApi/Schema/GeneratorSpec.hs
+++ b/test/Data/OpenApi/Schema/GeneratorSpec.hs
@@ -68,7 +68,7 @@ spec = do
     prop "T.Text" $ shouldValidate (Proxy :: Proxy T.Text)
     prop "TL.Text" $ shouldValidate (Proxy :: Proxy TL.Text)
     prop "[String]" $ shouldValidate (Proxy :: Proxy [String])
-    -- prop "(Maybe [Int])" $ shouldValidate (Proxy :: Proxy (Maybe [Int]))
+    prop "(Maybe [Int])" $ shouldValidate (Proxy :: Proxy (Maybe [Int]))
     prop "(IntMap String)" $ shouldValidate (Proxy :: Proxy (IntMap String))
     prop "(Set Bool)" $ shouldValidate (Proxy :: Proxy (Set Bool))
     prop "(NonEmpty Bool)" $ shouldValidate (Proxy :: Proxy (NonEmpty Bool))
diff --git a/test/Data/OpenApi/Schema/ValidationSpec.hs b/test/Data/OpenApi/Schema/ValidationSpec.hs
index 8b66189d..981e6ced 100644
--- a/test/Data/OpenApi/Schema/ValidationSpec.hs
+++ b/test/Data/OpenApi/Schema/ValidationSpec.hs
@@ -1,6 +1,7 @@
 {-# LANGUAGE CPP                 #-}
 {-# LANGUAGE DeriveGeneric       #-}
 {-# LANGUAGE PackageImports      #-}
+{-# LANGUAGE QuasiQuotes         #-}
 {-# LANGUAGE RecordWildCards     #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# OPTIONS_GHC -fno-warn-orphans #-}
@@ -8,11 +9,13 @@ module Data.OpenApi.Schema.ValidationSpec where
 
 import           Control.Applicative
 import           Control.Lens                        ((&), (.~), (?~))
+import           Control.Monad
 import           Data.Aeson
 #if MIN_VERSION_aeson(2,0,0)
 import qualified Data.Aeson.Key as Key
 import qualified Data.Aeson.KeyMap as KeyMap
 #endif
+import           Data.Aeson.QQ.Simple
 import           Data.Aeson.Types
 import           Data.Hashable                       (Hashable)
 import           Data.HashMap.Strict                 (HashMap)
@@ -45,6 +48,13 @@ import           Test.QuickCheck.Instances           ()
 shouldValidate :: (ToJSON a, ToSchema a) => Proxy a -> a -> Bool
 shouldValidate _ x = validateToJSON x == []
 
+shouldValidateValue :: (ToSchema a) => Proxy a -> Value -> Expectation
+shouldValidateValue px val = do
+  let (defs, sch) = runDeclare (declareSchema px) mempty
+  case validateJSON defs sch val of
+    [] -> pure ()
+    errors -> expectationFailure $ unlines errors
+
 shouldNotValidate :: forall a. ToSchema a => (a -> Value) -> a -> Bool
 shouldNotValidate f = not . null . validateJSON defs sch . f
   where
@@ -75,7 +85,8 @@ spec = do
     prop "T.Text" $ shouldValidate (Proxy :: Proxy T.Text)
     prop "TL.Text" $ shouldValidate (Proxy :: Proxy TL.Text)
     prop "[String]" $ shouldValidate (Proxy :: Proxy [String])
-    -- prop "(Maybe [Int])" $ shouldValidate (Proxy :: Proxy (Maybe [Int]))
+    prop "(Maybe [Int])" $ shouldValidate (Proxy :: Proxy (Maybe [Int]))
+    prop "(Maybe (Maybe Int))" $ shouldValidate (Proxy :: Proxy (Maybe (Maybe Int)))
     prop "(IntMap String)" $ shouldValidate (Proxy :: Proxy (IntMap String))
     prop "(Set Bool)" $ shouldValidate (Proxy :: Proxy (Set Bool))
     prop "(NonEmpty Bool)" $ shouldValidate (Proxy :: Proxy (NonEmpty Bool))
@@ -92,7 +103,11 @@ spec = do
     prop "(Int, String, Double)" $ shouldValidate (Proxy :: Proxy (Int, String, Double))
     prop "(Int, String, Double, [Int])" $ shouldValidate (Proxy :: Proxy (Int, String, Double, [Int]))
     prop "(Int, String, Double, [Int], Int)" $ shouldValidate (Proxy :: Proxy (Int, String, Double, [Int], Int))
-    prop "Person" $ shouldValidate (Proxy :: Proxy Person)
+    describe "Person: record with optional field" $ do
+      let px = Proxy :: Proxy Person
+      it "optional field is Just" $ shouldValidateValue px personJustEmailField
+      it "optional field is Null" $ shouldValidateValue px personNullEmailField
+      it "optional field is omitted" $ shouldValidateValue px personOmittedEmailField
     prop "Color" $ shouldValidate (Proxy :: Proxy Color)
     prop "Paint" $ shouldValidate (Proxy :: Proxy Paint)
     prop "MyRoseTree" $ shouldValidate (Proxy :: Proxy MyRoseTree)
@@ -128,6 +143,32 @@ instance ToSchema Person
 instance Arbitrary Person where
   arbitrary = Person <$> arbitrary <*> arbitrary <*> arbitrary
 
+personJustEmailField :: Value
+personJustEmailField = [aesonQQ|
+  {
+    "name" : "foo",
+    "phone" : 1,
+    "email" : "foo@email.com"
+  }
+|]
+
+personNullEmailField :: Value
+personNullEmailField = [aesonQQ|
+  {
+    "name" : "foo",
+    "phone" : 1,
+    "email" : null
+  }
+|]
+
+personOmittedEmailField :: Value
+personOmittedEmailField = [aesonQQ|
+  {
+    "name" : "foo",
+    "phone" : 1
+  }
+|]
+
 invalidPersonToJSON :: Person -> Value
 invalidPersonToJSON Person{..} = object
   [ stringToKey "personName"  .= toJSON name