Problem with TH

I am using the new existential datatype support (great!) -  but now when 
I patch in fundep support
(more or less as I posted before) adding an extra field to ClassD, I 
have to modify GHC to use the
extra field in ClassD... Now when I include a definition:

    [d| class Test x |]

I get the error:

ghc-6.3: panic! (the `impossible' happened, GHC version 6.3):
        deSugar/DsMeta.hs:925:4-41: Irrefutable pattern failed for 
pattern [elt_ty]

Any idea why this might be happening (why is the error in DsMeta, when 
the TH
stuff is in hsSyn/Convert.lhs and typecheck/TcSplice.lhs?

    Keean.