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?