Help with cast error

[Conal Elliott]

I'd appreciate help with a cast-related Core Lint error I'm getting with a
GHC plugin I'm working on:

    Argument value doesn't match argument type:
    Fun type:
        Enc (Vec ('S 'Z) Bool) ~ (Bool, ()) =>
        EP (Enc (Vec ('S 'Z) Bool)) -> EP (Bool, ())
    Arg type:
        ~R# (Enc (Vec ('S 'Z) Bool)) (Bool, ())
    Arg:
        CO Sub (TFCo:R:EncVec[0] <'Z>_N <Bool>_N)
           ; (Sub TFCo:R:EncBool[0], Sub (TFCo:R:EncVec0[0] <Bool>_N))_R

(I omitted the module prefixes for brevity.) Do I have a role wrong here,
or maybe something more fundamental?

I can easily supply more info if it'd help.

Thanks, -- Conal
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