diff --git a/mode/haskell/haskell.js b/mode/haskell/haskell.js
index 59ca7f0baaf28deab8c7e2fc3f969aab1a53f38c..68a6317e640cd77034bae755f44547e609dab245 100644
--- a/mode/haskell/haskell.js
+++ b/mode/haskell/haskell.js
@@ -237,7 +237,7 @@ CodeMirror.defineMode("haskell", function(_config, modeConfig) {
     token: function(stream, state) {
       var t = state.f(stream, function(s) { state.f = s; });
       var w = stream.current();
-      return (w in wellKnownWords) ? wellKnownWords[w] : t;
+      return wellKnownWords.hasOwnProperty(w) ? wellKnownWords[w] : t;
     },
 
     blockCommentStart: "{-",