Constraint depth cut off

Hi

I get the error message:

ERROR -
*** The type checker has reached the cutoff limit while trying to
*** determine whether:
***     Typeable (Foo Expr)
*** can be deduced from:
***     ()
*** This may indicate that the problem is undecidable.  However,
*** you may still try to increase the cutoff limit using the -c
*** option and then try again.  (The current setting is -c100)

I find this surprising because in the same file I have:

typename_Expr = mkTyCon "Expr"
instance Typeable Expr where
    typeOf _ = mkTyConApp typename_Expr []

typename_Foo = mkTyCon "Foo"
instance Typeable1 Foo where { typeOf1 _ = mkTyConApp typename_Foo [] }
instance Typeable a => Typeable (Foo a) where { typeOf = typeOfDefault }

I then went even further, giving it an explicit instance with:

instance Typeable (Foo Expr) where

And still I get the same error message. I originally had -c40
(default), but bumping it up has no effect. Any idea what may be
causing this?

Thanks

Neil