Troubleshooting undefined post properties error in Next.js with fallback: true

export const postQuery = groq`
  {
    'post': *[_type == "post" && slug.current == $slug] | order(_updatedAt desc)[0] {
      ${postFields}
    }
  }
`

export const postSlugsQuery = `
*[_type == "post" && defined(slug.current)][].slug.current
`

export const getStaticPaths: GetStaticPaths = async () => {
  const posts = await sanityClient.fetch(postSlugsQuery)

  return {
    paths: posts.map((slug: string) => ({ params: { slug } })),
    fallback: true,
  }
}

export const getStaticProps: GetStaticProps = async ({ params }) => {
  if (!params) throw new Error('No path parameters found')

  const { slug } = params
  const { post } = await sanityClient.fetch(postQuery, { slug })

  if (!post) {
    return {
      notFound: true,
      revalidate: 10,
    }
  }

  return {
    props: {
      post,
    },
    revalidate: 3600,
  }
}

Error occurred prerendering page "/[slug]". Read more: <https://nextjs.org/docs/messages/prerender-error>
TypeError: Cannot read properties of undefined (reading 'mainImage')
    at Post (/Users/jamessingleton/Code/Projects/redshirt-sports/.next/server/pages/[slug].js:44:40)
    at Wc (/Users/jamessingleton/Code/Projects/redshirt-sports/node_modules/react-dom/cjs/react-dom-server.browser.production.min.js:68:44)
    at Zc (/Users/jamessingleton/Code/Projects/redshirt-sports/node_modules/react-dom/cjs/react-dom-server.browser.production.min.js:70:253)
    at Z (/Users/jamessingleton/Code/Projects/redshirt-sports/node_modules/react-dom/cjs/react-dom-server.browser.production.min.js:76:89)
    at $c (/Users/jamessingleton/Code/Projects/redshirt-sports/node_modules/react-dom/cjs/react-dom-server.browser.production.min.js:78:98)
    at bd (/Users/jamessingleton/Code/Projects/redshirt-sports/node_modules/react-dom/cjs/react-dom-server.browser.production.min.js:77:404)
    at Z (/Users/jamessingleton/Code/Projects/redshirt-sports/node_modules/react-dom/cjs/react-dom-server.browser.production.min.js:76:217)
    at $c (/Users/jamessingleton/Code/Projects/redshirt-sports/node_modules/react-dom/cjs/react-dom-server.browser.production.min.js:78:98)
    at bd (/Users/jamessingleton/Code/Projects/redshirt-sports/node_modules/react-dom/cjs/react-dom-server.browser.production.min.js:77:404)
    at Z (/Users/jamessingleton/Code/Projects/redshirt-sports/node_modules/react-dom/cjs/react-dom-server.browser.production.min.js:76:217)

posts.map((post: Post) => console.log(post.slug))