NestJs: Make sure your class is decorated with an appropriate decorator

I am using graphql-request as a GraphQL client to query a headless CMS to fetch stuff, modify and return to the original request/query. headless cms is hosted separately fyi.

I have the following code :

@Query(returns => BlogPost)
  async test() {
    const endpoint = 'https://contentxx.com/api/content/project-dev/graphql'
    const graphQLClient = new GraphQLClient(endpoint, {
      headers: {
        authorization: 'Bearer xxxxxxx',
      },
    })
    const query = gql`
      {
        findContentContent(id: "9f5dde89-7f9b-4b9c-8669-1f0425b2b55d") {
          id
          flatData {
            body
            slug
            subtitle
            title
          }
        }
      }`

    return await graphQLClient.request(query);
  }

BlogPost is a model having the types :

import { Field, ObjectType } from '@nestjs/graphql';
import { BaseModel } from './base.model';
import FlatDateType from '../resolvers/blogPost/types/flatDatatype.type';

@ObjectType()
export class BlogPost extends BaseModel {
  @Field({ nullable: true })
  id!: string;

  @Field((type) => FlatDateType)
  flatData: FlatDateType;
}

and FlatDateType has the following code

export default class FlatDateType {
  body: string;
  slug: string;
  subtitle: string;
  title: string;
}

it throws the following exception :

Error: Cannot determine a GraphQL output type for the "flatData". Make sure your class is decorated with an appropriate decorator.

What is missing in here?