Trouble with random numbers in Haskell

I now have the following code:

func = do g <- newStdGen
          randomHeight <- fst (randomR (0, 700) g)
          rand10       <- fst (randomR (0, 10) g)
          randomSpeed  <- fst (randomR (2, 5) g)
          ...

but i get all kinds of error regarding the random integer generation. Any help on how to solve this would be greatly appreciated!!