diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,9 @@
 # Revision history for deriving-aeson
 
+## 0.2.3
+
+* Fixed a bug in `SumTaggedObject`
+
 ## 0.2.2
 
 * Added `UnwrapUnaryRecords`
diff --git a/deriving-aeson.cabal b/deriving-aeson.cabal
--- a/deriving-aeson.cabal
+++ b/deriving-aeson.cabal
@@ -1,6 +1,6 @@
 cabal-version:       2.4
 name:                deriving-aeson
-version:             0.2.2
+version:             0.2.3
 synopsis:            Type driven generic aeson instance customisation
 description:         This package provides a newtype wrapper with
   FromJSON/ToJSON instances customisable via a phantom type parameter.
@@ -13,6 +13,7 @@
 copyright:           Copyright (c) 2020 Fumiaki Kinoshita
 category:            JSON, Generics
 extra-source-files:  CHANGELOG.md, README.md
+tested-with:         GHC == 8.6.5, GHC == 8.8.3
 
 source-repository head
   type: git
diff --git a/src/Deriving/Aeson.hs b/src/Deriving/Aeson.hs
--- a/src/Deriving/Aeson.hs
+++ b/src/Deriving/Aeson.hs
@@ -72,6 +72,7 @@
 -- | the encoding will always follow the 'sumEncoding'.
 data NoAllNullaryToStringTag
 
+-- | Unpack single-field records
 data UnwrapUnaryRecords
 
 -- | Strip prefix @t@. If it doesn't have the prefix, keep it as-is.
@@ -100,9 +101,16 @@
 instance StringModifier CamelToSnake where
   getStringModifier = camelTo2 '_'
 
+-- | @{ "tag": t, "content": c}@
 data SumTaggedObject t c
+
+-- | @CONTENT@
 data SumUntaggedValue
+
+-- | @{ TAG: CONTENT }@
 data SumObjectWithSingleField
+
+-- | @[TAG, CONTENT]@
 data SumTwoElemArray
 
 -- | Reify 'Options' from a type-level list
@@ -131,7 +139,7 @@
   aesonOptions = (aesonOptions @xs) { allNullaryToStringTag = False }
 
 instance (KnownSymbol t, KnownSymbol c, AesonOptions xs) => AesonOptions (SumTaggedObject t c ': xs) where
-  aesonOptions = (aesonOptions @xs) { sumEncoding = TaggedObject (symbolVal (Proxy @ t)) (symbolVal (Proxy @ t)) }
+  aesonOptions = (aesonOptions @xs) { sumEncoding = TaggedObject (symbolVal (Proxy @ t)) (symbolVal (Proxy @ c)) }
 
 instance (AesonOptions xs) => AesonOptions (SumUntaggedValue ': xs) where
   aesonOptions = (aesonOptions @xs) { sumEncoding = UntaggedValue }