diff --git a/examples/RFC2616.txt b/examples/RFC2616.txt
new file mode 100644
--- /dev/null
+++ b/examples/RFC2616.txt
@@ -0,0 +1,6 @@
+GET http://slashdot.org/ HTTP/1.1
+foo: this is a test
+
+GET http://slashdot.org/ HTTP/1.0
+foo: of the emergency broadcast system
+
diff --git a/src/Text/Trifecta/Combinators.hs b/src/Text/Trifecta/Combinators.hs
--- a/src/Text/Trifecta/Combinators.hs
+++ b/src/Text/Trifecta/Combinators.hs
@@ -67,7 +67,7 @@
   restOfLine = Strict.drop . fromIntegral . columnByte <$> position <*> line
   {-# INLINE restOfLine #-}
 
-instance (MonadPlus m, DeltaParsing m) => DeltaParsing (Lazy.StateT s m) where
+instance (MonadPlus m, DeltaParsing m, Show s) => DeltaParsing (Lazy.StateT s m) where
   line = lift line
   {-# INLINE line #-}
   position = lift position
@@ -79,7 +79,7 @@
   restOfLine = lift restOfLine
   {-# INLINE restOfLine #-}
 
-instance (MonadPlus m, DeltaParsing m) => DeltaParsing (Strict.StateT s m) where
+instance (MonadPlus m, DeltaParsing m, Show s) => DeltaParsing (Strict.StateT s m) where
   line = lift line
   {-# INLINE line #-}
   position = lift position
@@ -103,7 +103,7 @@
   restOfLine = lift restOfLine
   {-# INLINE restOfLine #-}
 
-instance (MonadPlus m, DeltaParsing m, Monoid w) => DeltaParsing (Strict.WriterT w m) where
+instance (MonadPlus m, DeltaParsing m, Monoid w, Show w) => DeltaParsing (Strict.WriterT w m) where
   line = lift line
   {-# INLINE line #-}
   position = lift position
@@ -115,7 +115,7 @@
   restOfLine = lift restOfLine
   {-# INLINE restOfLine #-}
 
-instance (MonadPlus m, DeltaParsing m, Monoid w) => DeltaParsing (Lazy.WriterT w m) where
+instance (MonadPlus m, DeltaParsing m, Monoid w, Show w) => DeltaParsing (Lazy.WriterT w m) where
   line = lift line
   {-# INLINE line #-}
   position = lift position
@@ -127,7 +127,7 @@
   restOfLine = lift restOfLine
   {-# INLINE restOfLine #-}
 
-instance (MonadPlus m, DeltaParsing m, Monoid w) => DeltaParsing (Lazy.RWST r w s m) where
+instance (MonadPlus m, DeltaParsing m, Monoid w, Show s, Show w) => DeltaParsing (Lazy.RWST r w s m) where
   line = lift line
   {-# INLINE line #-}
   position = lift position
@@ -139,7 +139,7 @@
   restOfLine = lift restOfLine
   {-# INLINE restOfLine #-}
 
-instance (MonadPlus m, DeltaParsing m, Monoid w) => DeltaParsing (Strict.RWST r w s m) where
+instance (MonadPlus m, DeltaParsing m, Monoid w, Show s, Show w) => DeltaParsing (Strict.RWST r w s m) where
   line = lift line
   {-# INLINE line #-}
   position = lift position
@@ -201,13 +201,13 @@
   -- | Seek a previously marked location
   release :: d -> m ()
 
-instance (MonadPlus m, MarkParsing d m) => MarkParsing d (Lazy.StateT s m) where
+instance (MonadPlus m, MarkParsing d m, Show s) => MarkParsing d (Lazy.StateT s m) where
   mark = lift mark
   {-# INLINE mark #-}
   release = lift . release
   {-# INLINE release #-}
 
-instance (MonadPlus m, MarkParsing d m) => MarkParsing d (Strict.StateT s m) where
+instance (MonadPlus m, MarkParsing d m, Show s) => MarkParsing d (Strict.StateT s m) where
   mark = lift mark
   {-# INLINE mark #-}
   release = lift . release
@@ -219,25 +219,25 @@
   release = lift . release
   {-# INLINE release #-}
 
-instance (MonadPlus m, MarkParsing d m, Monoid w) => MarkParsing d (Strict.WriterT w m) where
+instance (MonadPlus m, MarkParsing d m, Monoid w, Show w) => MarkParsing d (Strict.WriterT w m) where
   mark = lift mark
   {-# INLINE mark #-}
   release = lift . release
   {-# INLINE release #-}
 
-instance (MonadPlus m, MarkParsing d m, Monoid w) => MarkParsing d (Lazy.WriterT w m) where
+instance (MonadPlus m, MarkParsing d m, Monoid w, Show w) => MarkParsing d (Lazy.WriterT w m) where
   mark = lift mark
   {-# INLINE mark #-}
   release = lift . release
   {-# INLINE release #-}
 
-instance (MonadPlus m, MarkParsing d m, Monoid w) => MarkParsing d (Lazy.RWST r w s m) where
+instance (MonadPlus m, MarkParsing d m, Monoid w, Show s, Show w) => MarkParsing d (Lazy.RWST r w s m) where
   mark = lift mark
   {-# INLINE mark #-}
   release = lift . release
   {-# INLINE release #-}
 
-instance (MonadPlus m, MarkParsing d m, Monoid w) => MarkParsing d (Strict.RWST r w s m) where
+instance (MonadPlus m, MarkParsing d m, Monoid w, Show s, Show w) => MarkParsing d (Strict.RWST r w s m) where
   mark = lift mark
   {-# INLINE mark #-}
   release = lift . release
diff --git a/src/Text/Trifecta/Parser.hs b/src/Text/Trifecta/Parser.hs
--- a/src/Text/Trifecta/Parser.hs
+++ b/src/Text/Trifecta/Parser.hs
@@ -148,6 +148,8 @@
   {-# INLINE unexpected #-}
   eof = notFollowedBy anyChar <?> "end of input"
   {-# INLINE eof #-}
+  notFollowedBy p = try (optional p >>= maybe (pure ()) (unexpected . show))
+  {-# INLINE notFollowedBy #-}
 
 instance Errable Parser where
   raiseErr e = Parser $ \ _ ee _ _ _ _ -> ee e
diff --git a/tests/QuickCheck.hs b/tests/QuickCheck.hs
new file mode 100644
--- /dev/null
+++ b/tests/QuickCheck.hs
@@ -0,0 +1,88 @@
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE CPP #-}
+
+module Main
+( main
+) where
+
+import Control.Applicative
+
+#if MIN_VERSION_base(4,7,0)
+import Data.Either
+#endif
+import Data.Monoid
+
+import qualified Test.QuickCheck as Q
+
+import Text.Parser.Char
+import Text.Parser.Combinators
+
+import Text.Trifecta.Parser
+import Text.Trifecta.Result
+
+import System.Exit
+
+-- -------------------------------------------------------------------------- --
+-- Main
+
+main :: IO ()
+main = mapM Q.quickCheckResult tests >>= \x -> case filter (not . passed) x of
+    [] -> exitSuccess
+    _ -> exitFailure
+  where
+    passed Q.Success{} = True
+    passed _ = False
+
+-- -------------------------------------------------------------------------- --
+-- Tests
+
+tests :: [Q.Property]
+tests =
+    [ Q.property prop_fail
+    , Q.property prop_succeed
+    , Q.property prop_notFollowedBy0
+    , Q.property prop_notFollowedBy1
+    , Q.property prop_notFollowedBy2
+    , Q.property prop_notFollowedBy3
+    ]
+
+-- -------------------------------------------------------------------------- --
+-- Properties
+
+prop_fail :: String -> Bool
+prop_fail = isLeft . parse (fail "fail" :: Parser ())
+
+prop_succeed :: String -> Bool
+prop_succeed = isRight . parse (mempty :: Parser ())
+
+prop_notFollowedBy0 :: Char -> Char -> Bool
+prop_notFollowedBy0 x y = either (\_ -> x == y) (/= y)
+    $ parse (notFollowedBy (char y) *> anyChar) [x]
+
+prop_notFollowedBy1 :: Char -> Bool
+prop_notFollowedBy1 x = either (\_ -> x == x) (/= x)
+    $ parse (notFollowedBy (char x) *> anyChar) [x]
+
+prop_notFollowedBy2 :: String -> Char -> Bool
+prop_notFollowedBy2 x y = isLeft
+    $ parse (anyChar *> notFollowedBy (char y) *> char y) x
+
+prop_notFollowedBy3 :: Char -> Bool
+prop_notFollowedBy3 x = isRight
+    $ parse (notFollowedBy (char x) <|> char x *> pure ()) [x]
+
+-- -------------------------------------------------------------------------- --
+-- Utils
+
+parse :: Parser a -> String -> Either String a
+parse p s = case parseString p mempty s of
+    Failure e -> Left (show e)
+    Success a -> Right a
+
+#if !MIN_VERSION_base(4,7,0)
+isLeft :: Either a b -> Bool
+isLeft = either (const True) (const False)
+
+isRight :: Either a b -> Bool
+isRight = either (const False) (const True)
+#endif
diff --git a/trifecta.cabal b/trifecta.cabal
--- a/trifecta.cabal
+++ b/trifecta.cabal
@@ -1,6 +1,6 @@
 name:          trifecta
 category:      Text, Parsing, Diagnostics, Pretty Printer, Logging
-version:       1.4.2
+version:       1.4.3
 license:       BSD3
 cabal-version: >= 1.10
 license-file:  LICENSE
@@ -16,7 +16,7 @@
 
 build-type:    Custom
 
-extra-source-files: examples/RFC2616.hs .travis.yml README.markdown
+extra-source-files: examples/RFC2616.hs examples/RFC2616.txt .travis.yml README.markdown
 
 source-repository head
   type: git
@@ -57,11 +57,11 @@
     ghc-prim,
     hashable             >= 1.2.1   && < 1.3,
     lens                 >= 4.0     && < 5,
-    mtl                  >= 2.0.1   && < 2.2,
+    mtl                  >= 2.0.1   && < 2.3,
     parsers              >= 0.10    && < 1,
     reducers             >= 3.10    && < 4,
     semigroups           >= 0.8.3.1 && < 1,
-    transformers         >= 0.2     && < 0.4,
+    transformers         >= 0.2     && < 0.5,
     unordered-containers >= 0.2.1   && < 0.3,
     utf8-string          >= 0.3.6   && < 0.4
 
@@ -84,3 +84,16 @@
 
   if impl(ghc<7.6.1)
     ghc-options: -Werror
+
+test-suite quickcheck
+  type:    exitcode-stdio-1.0
+  main-is: QuickCheck.hs
+  default-language: Haskell2010
+  build-depends:
+    base == 4.*,
+    parsers,
+    QuickCheck,
+    trifecta
+  ghc-options: -Wall -threaded
+  hs-source-dirs: tests
+