Typing issue with `ForeignKeyNullableRelation`

Describe the bug
Using mypy I get typing errors:

Incompatible types in assignment (expression has type "User | None", variable has type "ForeignKeyFieldInstance[Any] | None")
Cannot assign to attribute "solved_by" for class "Ticket"
  Type "User | None" is not assignable to type "ForeignKeyNullableRelation[User]"
    Type "User" is not assignable to type "ForeignKeyNullableRelation[User]"
      "User" is not assignable to "ForeignKeyFieldInstance[User]"
      "User" is not assignable to "None"
(parameter) solved_by: User | None

To Reproduce
I have (pretty much) the following code:

from tortoise.models import Model

class User(Model):
    id = fields.IntField(unique=True, pk=True)

class Ticket(Model):
    id = fields.IntField(unique=True, pk=True)

    solved_by : fields.ForeignKeyNullableRelation[User] = fields.ForeignKeyField("User", "solved_tickets", null=True)

And the following is causing the issue:

async def set_solved_by(self, solved_by: User | None): # self here is a wrapper class with a state, `model` is an instance of `Ticket`
    self.model.solved_by = solved_by 

Expected behavior
To not get typing errors

Additional context
I'm not quite sure if I'm using the correct typing for the field, but I assume I do?

Start by locating the definition of ForeignKeyNullableRelation and reproduce the mypy errors with the User and Ticket example in the issue. Trace the annotation used for the nullable relation and verify that assigning User or None to Ticket.solved_by type-checks without the reported errors.