[Haskell-beginners] What's wrong with Haddock?!

I often hit such errors: parse error on input ‘-- ^ my doc-string is here’ when generate Haddock documentation. This happens on line like: instance Something UTCTime where some a = blahBlah -- ^ my doc-string is here Sometimes such doc-strings are passing, sometimes - not. I can understand what is the reason. Is there some rule how to format such kind of doc-strings? === Cheers, Paul