Gatsby v3 Tutorial Part 6: slug in GraphQL query is undefined

[ngAlexander9]

I'm following along with the Gatsby v3 tutorial.

My src/pages/blog/index.js (identical to tutorial) should query the slug to generate the correct links, but the resulting links are http://localhost:8000/blog/undefined.

Manually running the query at http://localhost:8000/___graphql returns the expected slug values, but logging the query to console in src/pages/blog/index.js returns all the other values except slug.

src/pages/blog/index.js

import * as React from 'react'
import { Link, graphql } from 'gatsby'
import Layout from '../../components/layout'

const BlogPage = ({ data }) => {
  return (
    <Layout pageTitle="My Blog Posts">
      {
        data.allMdx.nodes.map(node => (
          <article key={node.id}>
            <h2>
              <Link to={`/blog/${node.slug}`}>
                {node.frontmatter.title}
              </Link>
            </h2>
            <p>Posted: {node.frontmatter.date}</p>
          </article>
        ))
      }
    </Layout>
  )
}

export const query = graphql`
  query {
    allMdx(sort: {fields: frontmatter___date, order: DESC}) {
      nodes {
        frontmatter {
          date(formatString: "MMMM D, YYYY")
          title
        }
        id
        slug
      }
    }
  }
`

export default BlogPage