GHCi suggestion for instance signature (InstanceSigs) is wrong

With InstanceSigs and PolyKinds enabled and the following type with kind ĸ -> *:

    data NullableInterp a = NullI Bool

I want to make it an instance of Alternative and I decide to supply a signature for empty:

    instance Alternative NullableInterp where
      empty :: NullableInterp a
      empty = undefined
      (<|>) = undefined

but GHCi complains with the following error:

% ghci -ignore-dot-ghci error.hs
GHCi, version 7.8.2: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main             ( error.hs, interpreted )

error.hs:14:12:
    Method signature does not match class; it should be
      empty :: forall a. NullableInterp a
    In the instance declaration for ‘Alternative NullableInterp’
Failed, modules loaded: none.
Prelude>

Trying GHCi's suggestion (with appropriate extensions) doesn't help and I have to write empty :: NullableInterp (a :: *) for it to work. I'm not sure which behaviour is the right one but either it should have compiled or GHCi's suggestion should be empty :: forall a. NullableInterp (a :: *).

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
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CC	hvr
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