Could it be that (Alternative f, Foldable f) => Monad f?

The following typechecks:

instance (Applicative f, Alternative f, Foldable f) => Monad f where 
  (>>=) = flip $ \f -> foldr (<|>) empty . fmap f
  -- Or equivalently
  a >>= b = getAlt . foldMap Alt . fmap b $ a

Is this actually a valid Monad instance? If yes, why is it not used? If no, does it break any laws or such? I have not proved that the laws hold, but I couldn't find a counterexample either.