What am I doing wrong in this admittedly contrived example?

The code below will compile. It works as expected, unless I try to do “toMyParam Nothing”. See below:

λ: let arr = [P1 3.0, P2 'x']

λ: toMyParam False

P2 'F'

λ: toMyParam (Just 'x')

P2 'x'

λ: toMyParam Nothing

<interactive>:38:1:

    No instance for (ToMyParam a0) arising from a use of `toMyParam'

    The type variable `a0' is ambiguous

Code below:

data MyParam = P1 Double | P2 Char deriving Show

class ToMyParam a where

  toMyParam :: a -> MyParam

instance ToMyParam Bool where

  toMyParam False = P2 'F'

  toMyParam True = P2 'T'

instance ToMyParam Char where

  toMyParam = P2

instance ToMyParam Double where

  toMyParam = P1

instance ToMyParam a => ToMyParam (Maybe a) where

  toMyParam Nothing = P1 0.0

  toMyParam (Just x) = toMyParam x