Mutation test failing with executeOperation (Apollo Graphql)

I want to test my graphql queries and mutations using Jest. I am writing my unit tests as given in Apollo docs:

describe('Tests', () => {
  let testServer;

  beforeAll(() => {
    testServer = new ApolloServer({ typeDefs, resolvers });
  });

  // Below test passes 
  it('should return matching result for the movies query', async () => {
    const limit = 3;
    const query = `
              query {
                movies(limit: ${limit}) {
                  id
                  name
                }
              }
            `;

    const result = await testServer.executeOperation({ query });
    expect(result.data.movies.length).toBe(limit);
  });

  // Below test fails.
  it('should mark movie as watched using the markWatched mutation', async () => {
    const movieIds = [9873, 8754];
    const mutation = `
                  mutation {
                      markWatched(movieIds: ${movieIds}) {
                      id,
                      watched
                    }
                  }
                `;

    const result = await testServer.executeOperation({ query: mutation });
    expect(result.data.markWatched).toBe([
      {
        id: 9873,
        watched: true
      },
      {
        id: 8754,
        watched: true
      }
    ]);
  });
});

// Mutation
 type Mutation {
    markWatched(ids: [ID]!): [Movies]!
 }

// Resolver
Mutation: 
{
    markWatched: (_, { ids }) => 
        markMoviesAsWatched(ids)
}

// Data model (which is simply a local file exporting an array - moviesData in below code)
const markMoviesAsWatched = ids => {
  const response = [];
  moviesData.forEach(movie => {
    if (ids.includes(movie.id)) {
      movie.watched = true;
      response.push(movie);
    }
  });
  return response;
};

The first test regarding query is passing while the second test regarding mutation is failing with error: TypeError: Cannot read properties of undefined (reading 'markWatched'). The exact mutation runs fine in Apollo playground. Not sure what's wrong with my mutation test?

from Mutation test failing with executeOperation (Apollo Graphql)