diff --git a/Text/Pandoc/Definition.hs b/Text/Pandoc/Definition.hs
--- a/Text/Pandoc/Definition.hs
+++ b/Text/Pandoc/Definition.hs
@@ -263,7 +263,7 @@
                         , Aeson.constructorTagModifier = id
                         , Aeson.allNullaryToStringTag = False
                         , Aeson.omitNothingFields = False
-                        , Aeson.sumEncoding = Aeson.ObjectWithSingleField
+                        , Aeson.sumEncoding = Aeson.TaggedObject "t" "c"
                         }
 
 toJSON' :: (Generic a, Aeson.GToJSON (Rep a))
diff --git a/pandoc-types.cabal b/pandoc-types.cabal
--- a/pandoc-types.cabal
+++ b/pandoc-types.cabal
@@ -1,5 +1,5 @@
 Name:                pandoc-types
-Version:             1.12.2.3
+Version:             1.12.3
 Synopsis:            Types for representing a structured document
 Description:         @Text.Pandoc.Definition@ defines the 'Pandoc' data
                      structure, which is used by pandoc to represent