Kind variables not allowed in type family instance declarations

I am trying to create a kind-indexed type family, but I cannot use kind variables in my kind annotations:

{-# LANGUAGE PolyKinds, TypeFamilies, DataKinds #-}

type family KindFam a
type instance KindFam (a :: *) = Int
type instance KindFam (a :: Bool) = Bool
type instance KindFam (a :: Maybe k) = Char -- doesn't work

Trac metadata

Trac field	Value
Version	7.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture