[Haskell-beginners] Adding 1 to Just 9

[Olumide]

Dear List,

Chapter 14 of LYH appears to suggest that a Just value can be added to 
an Int. Quote from 
http://learnyouahaskell.com/for-a-few-monads-more#useful-monadic-functions

"For instance, say we have Just 9 and the function \x -> Just (x+1). If 
we map this function over Just 9, we're left with Just (Just 10)."

I've tried the following in ghci but got the error:

<interactive>:12:1: error:
     • Non type-variable argument in the constraint: Num (Maybe a)
       (Use FlexibleContexts to permit this)
     • When checking the inferred type
         it :: forall a. (Num (Maybe a), Num a) => Maybe a


Am I reading the quote wrong? Is Just (Just 10) a hypothetical?

Regards,

- Olumide