Why is markdownRemark undefined in this scenario? – SEO

I’m very new to Gatsby (started with it yesterday) and have run into a problem. The app works perfectly in develop, but when trying to build I get this error:

failed Building static HTML for pages - 1.092s

 ERROR #95313

Building static HTML failed for path "/annihilation"

See our docs page for more info on this error: https://gatsby.dev/debug-html


  15 | }) {
  16 |  const { markdownRemark } = data // data.markdownRemark holds your post data
> 17 |  const { frontmatter, html } = markdownRemark
     |          ^
  18 |
  19 |  return (
  20 |      <Layout>


  WebpackError: TypeError: Cannot destructure property `frontmatter` of 'undefined' or 'null'.

  - index.js:17 Template
    src/templates/entryTemplate/index.js:17:10


error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
Command failed: /usr/local/Cellar/yvm/3.4.0/versions/v1.21.1/bin/yarn.js build

This is my config file:

module.exports = {
    siteMetadata: {
        title: `Best Horror Scenes — An ever growing collection featuring some of the best scenes in horror.`,
        description: `“Best Horror Scenes” is a collection of scenes I feel are some of the most affecting in horror. Some may be simple black cat scares, others may be more subdued or nuanced. Many come from films that aren’t necessarily “horror” but have elements or threads of horror, and all have the same general effect: unease, dread, fear, shock, disgust.`,
        ogImage: 'https://besthorrorscenes.com/images/social.png',
    },
    plugins: [
        'gatsby-plugin-react-helmet',
        'gatsby-plugin-postcss',
        'gatsby-transformer-sharp',
        'gatsby-plugin-sharp',
        'gatsby-transformer-remark',
        'gatsby-plugin-feed',
        {
            resolve: 'gatsby-plugin-google-analytics',
            options: {
                trackingId: 'UA-XXXXXXXX-XX',
            },
        },
        {
            resolve: 'gatsby-plugin-react-svg',
            options: {
                rule: {
                    include: /assets/,
                },
            },
        },
        {
            resolve: 'gatsby-source-filesystem',
            options: {
                name: 'entries',
                path: `${__dirname}/src/entries`,
            },
        },
        {
            resolve: 'gatsby-source-filesystem',
            options: {
                name: 'images',
                path: `${__dirname}/src/images`,
            },
        },
        {
            resolve: 'gatsby-plugin-manifest',
            options: {
                name: 'best-horror-scenes',
                short_name: 'best-horror-scenes',
                start_url: '/',
                background_color: '#d94439',
                theme_color: '#d94439',
                display: 'minimal-ui',
                icon: 'src/images/icon.png',
            },
        },
        // this (optional) plugin enables Progressive Web App + Offline functionality
        // To learn more, visit: https://gatsby.dev/offline
        `gatsby-plugin-offline`,
    ],
}

… and my node file:

const path = require(`path`)
exports.createPages = async ({ actions, graphql, reporter }) => {
    const { createPage } = actions
    const entryTemplate = path.resolve(`src/templates/entryTemplate/index.js`)
    const result = await graphql(`
        {
            allMarkdownRemark(
                sort: { order: DESC, fields: [frontmatter___index] }
                limit: 1000
            ) {
                edges {
                    node {
                        frontmatter {
                            path
                        }
                    }
                }
            }
        }
    `)

    // Handle errors
    if (result.errors) {
        reporter.panicOnBuild(`Error while running GraphQL query.`)
        return
    }

    result.data.allMarkdownRemark.edges.forEach(({ node }) => {
        createPage({
            path: node.frontmatter.path,
            component: entryTemplate,
            context: {}, // additional data can be passed via context
        })
    })
}

The complaint involves my entryTemplate file, which looks like this
:

import React from "react"
import { graphql, Link } from "gatsby"

import Article from '../../components/Article'
import Layout from '../../components/Layout'
import SEO from '../../components/seo'

import BackArrow from '../../assets/arrow.svg'

// Styles
import style from './index.module.css'

export default function Template({
    data,
}) {
    const { markdownRemark } = data
    const { frontmatter, html } = markdownRemark

    return (
        <Layout>
            <SEO
                image={ `https://besthorrorscenes.com/posters/${frontmatter.poster}` }
                title={ frontmatter.title }
                url={ `https://besthorrorscenes.com${frontmatter.path}` }
            />
            <nav className={ style.ArticleNav }>
                <Link className={ style.BackLink } to="/">
                    <BackArrow className={ style.BackArrow } />
                    Back to list
                </Link>
            </nav>
            <Article
                standalone
                director={ frontmatter.director }
                entryNumber={ frontmatter.index }
                isPlaying={ false }
                key={ frontmatter.index }
                poster={ frontmatter.poster }
                setIsPlaying={ () => {} }
                slug={ frontmatter.path }
                title={ frontmatter.title }
                url={ frontmatter.url }
                year={ frontmatter.year }
            />
            <section className={ style.DidYouKnow }>
                <h2>Did<br />You<br />Know?</h2>
                <div className={ style.DidYouKnowContent } dangerouslySetInnerHTML={ { __html: html } } />
            </section>
        </Layout>
    )
}

export const query = graphql`
    query($path: String!) {
        markdownRemark(frontmatter: { path: { eq: $path } }) {
            html
            frontmatter {
                director
                index
                path
                poster
                title
                url
                year
            }
        }
    }
`

I’m at a loss here since it works in develop mode, but I expect the error will be clear to the more experienced.

I appreciate any help I can get.

EDIT:

This actually does happen in develop mode once I go to any of the routes.