Why int list as unnamed type is not allowed in variant types?

[mryau]

Hello, I don't understand why following declarations will be compiled:
type is = int list [@@deriving protobuf { protoc }] type io = | Port of int [@key 1] | Agg of is [@key 2] [@@deriving protobuf { protoc }]
but this declaration:
type io = | Port of int [@key 1] | Agg of int list [@key 2] [@@deriving protobuf { protoc }]
will cause the following error:
File "src/ppx_deriving_protobuf.cppo.ml", line 1113, characters 28-34: Assertion failed

[whitequark]

An assertion failure is a bug.

[glyh]

Could anyone provide any hint on how should I fix it?