NestJS GraphQL auto-registration of interface fields when using interface resolvers doesn't work properly

Question

I would like to use interface resolver as described in official documentation of NestJS and I have created following code to try it out.

Models are in file models.ts:

import { Field, ID, InterfaceType, ObjectType } from '@nestjs/graphql';

@ObjectType()
export class Task {
  @Field((type) => ID)
  id: string;

  @Field()
  description: string; 
}

@InterfaceType()
export abstract class ITaskAssignable {
  @Field((type) => [Task])
  tasks: Task[]
}

@ObjectType({ implements: () => [ITaskAssignable] })
export class Person implements ITaskAssignable {
  @Field((type) => ID)
  id: string;

  @Field()
  name: string;

  @Field((type) => [Task])
  tasks: Task[]
}

Resolvers with mock logic are in file resolvers.ts:

import { Args, Info, Parent, Query, ResolveField, Resolver } from '@nestjs/graphql';
import { ITaskAssignable, Person, Task } from './models';

@Resolver(of => Task)
export class TaskResolver {
  constructor() { }

  @Query(returns => Task)
  async task(@Args('id') id: string): Promise<Task> {
    return { id, description: 'something' };
  }
}

@Resolver(of => Person)
export class PersonResolver {
  constructor() { }

  @Query(returns => Person)
  async person(@Args('id') id: string): Promise<Person> {
    return { id, name: 'John Doe' } as Person;
  }
}

@Resolver(of => ITaskAssignable)
export class ITaskAssignableResolver {
  @ResolveField(returns => [Task])
  async tasks(@Parent() parent, @Info() info): Promise<Task[]> {
    return [{ id: 'example-id', description: 'something' }];
  }
}

App module and Task module are in one file modules.ts:

import { ApolloDriver, ApolloDriverConfig } from '@nestjs/apollo';
import { Module } from '@nestjs/common';
import { GraphQLModule } from '@nestjs/graphql';
import { PersonResolver, TaskResolver } from './resolvers';

@Module({
  providers: [PersonResolver, TaskResolver],
})
export class TaskModule { }

@Module({
  imports: [
    TaskModule,
    GraphQLModule.forRoot<ApolloDriverConfig>({
      driver: ApolloDriver,
      autoSchemaFile: 'schema.gql',
    }),
  ],
})
export class AppModule { }

Last file is main.ts:

import { ValidationPipe } from '@nestjs/common';
import { NestFactory } from '@nestjs/core';
import { AppModule } from './modules';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  app.useGlobalPipes(new ValidationPipe());

  await app.listen(3000, 'localhost');
  console.log(`Application is running on: ${await app.getUrl()}`);
}
bootstrap();

Thetasks field is auto-registered for Person model and can be queried but throws error: "Cannot return null for non-nullable field Person.tasks.",

I have several times read through documentation and tried to find if someone encounter similar problem, but I didn't found solution.