ModelGenerator fails to generate model with primitive type wrapped in object

[genme]

import json

from dataclasses_avroschema import ModelGenerator

schema = """
{
  "namespace": "Foo.Boo",
  "name": "Zoo",
  "type": "record",
  "fields": [
    {
      "name": "value_a",
      "type": [
        "null", {"type": "string"}
      ]
    },
    {
      "name": "value_b",
      "type": [
        "null",
        "string"
      ]
    }
  ]
}
"""

mg = ModelGenerator()
print(mg.render(schema=json.loads(schema)))

# from dataclasses_avroschema import AvroModel
# import dataclasses
# import typing
#
#
# @dataclasses.dataclass
# class Zoo(AvroModel):
#     value_a: typing.Optional[: str] <------------------------
#     value_b: typing.Optional[str]
#
#     class Meta:
#         namespace = "Foo.Boo"