Cannot compile the mutation

[pnkapadia64]

I'm trying to work out a mutation to remove a favourite game corresponding to a user's email address. The input is pretty straightforward, two strings. I keep getting the error
NoExplicitTypeError: Unable to infer GraphQL type from TypeScript reflection system. You need to provide explicit type for 'removeUserFavouriteGameMutation' of 'FavouriteGamesResolver' class.
The error is same even if the input doesn't extend an object (here FavouriteUserGame) or if I use an abstract class. Not sure what is causing it.

@ObjectType()
class FavouriteUserGame {
  @Field()
  email: string;
  @Field()
  gameSlug: string;
}

@InputType()
class FavouriteGameInput implements Partial<FavouriteUserGame> {
  @Field()
  email: string;
  @Field()
  gameSlug: string;
}

@Resolver()
export class FavouriteGamesResolver {
  @Mutation()
  @Authorized()
  async removeUserFavouriteGameMutation(@Args() { email, gameSlug }: FavouriteGameInput, @Ctx('dataSources') dataSources: UPDataSources) {
    // mutation
    return;
  }
}

[MichalLytek]

@Mutation(type => SclaraOrObjectTypeClassYouReturn)

If you want to return undefined or void, in GraphQL you should return true; and use Boolean scalar type.