Function not_a_handle_error 

fn not_a_handle_error(actual: SEXPTYPE) -> SexpError

Error for an ExternalPtr<T> argument that is neither a bare EXTPTRSXP nor a class handle wrapping one (audit A9 — class-wrapped handles like Foo$new(...) are unwrapped automatically; this fires only when no .ptr/slot/attribute could be recovered at all).