diff --git a/JsonGrammar.cabal b/JsonGrammar.cabal
--- a/JsonGrammar.cabal
+++ b/JsonGrammar.cabal
@@ -1,5 +1,5 @@
 Name:           JsonGrammar
-Version:        0.3.2
+Version:        0.3.3
 Synopsis:       Combinators for bidirectional JSON parsing
 Description:   	Combinators for bidirectional JSON parsing
 
@@ -7,7 +7,7 @@
 Author:         Martijn van Steenbergen
 Maintainer:     martijn@van.steenbergen.nl
 Stability:      Experimental
-Copyright:      Some Rights Reserved (CC) 2010-2011 Martijn van Steenbergen
+Copyright:      Some Rights Reserved (CC) 2010-2012 Martijn van Steenbergen
 Homepage:       https://github.com/MedeaMelana/JsonGrammar
 Bug-reports:    https://github.com/MedeaMelana/JsonGrammar/issues
 
@@ -26,15 +26,15 @@
                     Data.Iso.Common,
                     Language.JsonGrammar
   Build-Depends:    base >= 3.0 && < 4.6,
-                    containers >= 0.3 && < 0.5,
                     aeson >= 0.6 && < 0.7,
                     semigroups >= 0.5 && < 0.9,
-                    template-haskell >= 2.4 && < 2.8,
-                    -- constraints copied from aeson-0.6.0.0:
+                    -- constraints copied from aeson-0.6.1.0:
                     attoparsec >= 0.8.6.1,
                     bytestring,
+                    containers,
                     hashable >= 1.1.2.0,
                     text >= 0.11.0.2,
+                    template-haskell >= 2.4,
                     time,
                     unordered-containers >= 0.1.3.0,
                     vector >= 0.7.1
diff --git a/Language/JsonGrammar.hs b/Language/JsonGrammar.hs
--- a/Language/JsonGrammar.hs
+++ b/Language/JsonGrammar.hs
@@ -26,7 +26,7 @@
 import Data.IntSet (IntSet)
 import Data.Iso hiding (option)
 import qualified Data.HashMap.Lazy as M
-import Data.Maybe (fromMaybe, isJust)
+import Data.Maybe (fromMaybe, isNothing)
 import Data.String
 import Data.Text (Text)
 import qualified Data.Text.Lazy as Lazy
@@ -133,7 +133,7 @@
 
 -- Defined in Data.Map but not in Data.HashMap.Lazy:
 notMember :: (Eq k, Hashable k) => k -> M.HashMap k v -> Bool
-notMember k m = isJust (M.lookup k m)
+notMember k m = isNothing (M.lookup k m)
 
 -- | Collect all properties left in an object.
 rest :: Iso (Object :- t) (Object :- M.HashMap Text Value :- t)