...
if (sv_isobject(ST(0)) && sv_derived_from(ST(0), "Audio::TagLib::ID3v2::Fram
        THIS = INT2PTR(TagLib::ID3v2::Frame *, SvIV((SV *)SvRV(ST(0))));
    else
        Perl_croak(aTHX_ "THIS is not of type Audio::TagLib::ID3v2::Frame");