diff --git a/haddock-library/src/Documentation/Haddock/Parser/Monad.hs b/haddock-library/src/Documentation/Haddock/Parser/Monad.hs
index a421c58c58844142e370ddd907e989b684aa8460..3f7d60f840cc99c40fd4e7baa86e2f976d8eae08 100644
--- a/haddock-library/src/Documentation/Haddock/Parser/Monad.hs
+++ b/haddock-library/src/Documentation/Haddock/Parser/Monad.hs
@@ -42,7 +42,7 @@ import           Data.Tuple
 
 import           Documentation.Haddock.Types (Version)
 
-data ParserState = ParserState {
+newtype ParserState = ParserState {
   parserStateSince :: Maybe Version
 } deriving (Eq, Show)