diff --git a/Data/Aeson.hs b/Data/Aeson.hs
--- a/Data/Aeson.hs
+++ b/Data/Aeson.hs
@@ -68,6 +68,7 @@
     , withText
     , withArray
     , withNumber
+    , withScientific
     , withBool
     -- * Constructors and accessors
     , (.=)
diff --git a/Data/Aeson/TH.hs b/Data/Aeson/TH.hs
--- a/Data/Aeson/TH.hs
+++ b/Data/Aeson/TH.hs
@@ -165,7 +165,7 @@
   where
     fromCons :: [TyVarBndr] -> [Con] -> Q Dec
     fromCons tvbs cons =
-        instanceD (return $ map (\t -> ClassP ''ToJSON [VarT t]) typeNames)
+        instanceD (applyCon ''ToJSON typeNames)
                   (classType `appT` instanceType)
                   [ funD 'toJSON
                          [ clause []
@@ -372,7 +372,7 @@
   where
     fromCons :: [TyVarBndr] -> [Con] -> Q Dec
     fromCons tvbs cons =
-        instanceD (return $ map (\t -> ClassP ''FromJSON [VarT t]) typeNames)
+        instanceD (applyCon ''FromJSON typeNames)
                   (classType `appT` instanceType)
                   [ funD 'parseJSON
                          [ clause []
@@ -872,3 +872,12 @@
 valueConName (Number _) = "Number"
 valueConName (Bool   _) = "Boolean"
 valueConName Null       = "Null"
+
+applyCon :: Name -> [Name] -> Q [Pred]
+applyCon con typeNames = return (map apply typeNames)
+  where apply t =
+#if __GLASGOW_HASKELL__ >= 709
+          AppT (ConT con) (VarT t)
+#else
+          ClassP con [VarT t]
+#endif
diff --git a/Data/Aeson/Types/Instances.hs b/Data/Aeson/Types/Instances.hs
--- a/Data/Aeson/Types/Instances.hs
+++ b/Data/Aeson/Types/Instances.hs
@@ -107,7 +107,10 @@
     parseJSON (Object (H.toList -> [(key, value)]))
         | key == left  = Left  <$> parseJSON value
         | key == right = Right <$> parseJSON value
-    parseJSON _        = fail ""
+    parseJSON _        = fail $
+        "expected an object with a single property " ++
+        "where the property key should be either " ++
+        "\"Left\" or \"Right\""
     {-# INLINE parseJSON #-}
 
 left, right :: Text
diff --git a/aeson.cabal b/aeson.cabal
--- a/aeson.cabal
+++ b/aeson.cabal
@@ -1,5 +1,5 @@
 name:            aeson
-version:         0.7.0.2
+version:         0.7.0.3
 license:         BSD3
 license-file:    LICENSE
 category:        Text, Web, JSON