more undecidable instances, but why

[Hal Daume III]

I'm being hit by undecidable instances, but I can't see why.  I read and
reread the GHC docs on MPTCs, but to no avail.  This is what I'm trying to
do:

  newtype S a = S a
  instance MArray IOUArray a IO => MArray IOUArray (S a) IO where
    ...

the instance declaration is basically just wrapping/unwrapping S and a few
castIOArrays.

However, GHC complains:

/nfs/isd/hdaume/projects/MESumm/Linear2/Linear2.hs:76:
    Non-type variables in constraint: MArray IOUArray a IO
    (Use -fallow-undecidable-instances to permit this)
    In the context: (MArray IOUArray a IO)
    While checking the context of an instance declaration
    In the instance declaration for `MArray IOUArray (S a) IO'

But I don't see why this is undecidable.  As I understand it, undecidable
instances basically arise when context reduction might not
terminate.  Here, it always will (AFAICS).

Any thoughts?

 - Hal