GraphQL query returns error "Cannot return null for non-nullable field"

I have a basic GraphQL query setup as follows:

Query.js:

const Query = {
    dogs(parent, args, ctx, info) {
        return [{ name: 'Snickers' }, { name: 'Sunny' }];
    },
};

module.exports = Query;

schema.graphql:

type Dog {
    name: String!
}
type Query {
    dogs: [Dog]!
}

I created a function createServer() for starting the server as follows:

const { GraphQLServer } = require('graphql-yoga');
const Mutation = require('./resolvers/Mutation');
const Query = require('./resolvers/Query');
const db = require('./db');

function createServer() {
    return new GraphQLServer({
        typeDefs: 'src/schema.graphql',
        resolvers: {
            Mutation,
            Query,
        },
        resolverValidationOptions: {
            requireResolversForResolveType: false,
        },
        context: req => ({ ...req, db }),
    });
}

module.exports = createServer;

I then tried querying dogs as follows:

query {
  dogs {
    name
  }
}

But instead of getting the names from the array of dogs, I got the following error instead:

{
  "data": null,
  "errors": [
    {
      "message": "Cannot return null for non-nullable field Query.dogs.",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "dogs"
      ]
    }
  ]
}

What seems to be causing this error?