Anomaly with unipoly, index inductive and number notations

[Alizter]

Set Universe Polymorphism.
Axiom A : Type.

Inductive B : A -> Type :=
| x {a} : B a
| y {a} : B a -> B a
.

Number Notation B Nat.of_num_uint Nat.to_num_uint
  (via nat mapping [[x] => O, [y] => S]) : nat_scope.

Anomaly "bad instance length on mutind."
Please report at http://coq.inria.fr/bugs/.

[Alizter]

cc @coq/universes-maintainers @coq/number-maintainers

[SkySkimmer]

I guess it does type_of_ind with a dummy empty instance.
Not sure if number notations are supposed to work with univ poly inductives, if not then there should be a check before this happens I suppose.