diff --git a/haddock-library.cabal b/haddock-library.cabal
--- a/haddock-library.cabal
+++ b/haddock-library.cabal
@@ -1,5 +1,5 @@
 name:                 haddock-library
-version:              1.4.1
+version:              1.4.2
 synopsis:             Library exposing some functionality of Haddock.
 description:          Haddock is a documentation-generation tool for Haskell
                       libraries. These modules expose some functionality of it
diff --git a/src/Documentation/Haddock/Parser/Monad.hs b/src/Documentation/Haddock/Parser/Monad.hs
--- a/src/Documentation/Haddock/Parser/Monad.hs
+++ b/src/Documentation/Haddock/Parser/Monad.hs
@@ -42,7 +42,7 @@
 
 import           Documentation.Haddock.Types (Version)
 
-data ParserState = ParserState {
+newtype ParserState = ParserState {
   parserStateSince :: Maybe Version
 } deriving (Eq, Show)