title: Graphql error on subfields with graphql-yoga
body: <p>I'm trying to query a graphql API via a proxy of another graphql API and am receiving an error. I'm using graphql yoga for my server and connecting to another graphql API from a CMS. Here is my code:</p>
  
  <h1>server.js</h1>
  
  <pre><code>const { GraphQLServer } = require('graphql-yoga');
  const Prismic = require('./prismic.config.js');
  const gql = require('graphql-tag');
  
  const typeDefs = `
    type Query {
      page(lang: String, uid: String): Page
    }
  
    type Page {
      page_title: [TextField]
    }
  
    type TextField {
      text: String
    }
  `
  
  const resolvers = {
    Query: {
      page: (parent, args, context, info) =&gt; {
        const query = gql`${context.request.body.query}`;
  
        const result = context.Prismic.query({
          query,
          variables: { ...args }
        })
        .then(resp =&gt; {
          return resp.data.page;
        })
        .catch(err =&gt; console.log(err));
        return result;
      }
    }
  }
  
  const server = new GraphQLServer({ 
    typeDefs, 
    resolvers,
    context: req =&gt; ({ ...req, Prismic })
  })
  
  server.start(() =&gt; console.log('Server is running on localhost:4000'))
  </code></pre>
  
  <p>Here is my query below from graphql playground that comes with Graphql Yoga:</p>
  
  <pre><code>query {
    page(lang: "en-gb", uid: "homepage") {
      page_title {
        text
      }
    }
  }
  </code></pre>
  
  <p>The error i'm receiving is:</p>
  
  <blockquote>
    <p>'Query does not pass validation. Violations:\n\nField \'page_title\'
    of type \'Json\' must not have a sub selection. (line 3, column 5):\n 
    page_title {\n    ^' } },</p>
  </blockquote>
  
  <p>The strange thing is I can get a valid response if I hardcode the query without the nested <code>text</code> field as the error suggests on the server like so:</p>
  
  <pre><code>// const query = gql`${context.request.body.query}`;
  
  const query = gql`
        query($uid: String!) {
          page(lang: "en-gb", uid: $uid) {
            page_title
          }
        }
      `;
  </code></pre>
  
  <p>Attempting to modify my query in graphql playground to not include the nested <code>text</code> field like so:</p>
  
  <pre><code>query {
    page(lang: "en-gb", uid: "homepage") {
      page_title
    }
  }
  </code></pre>
  
  <p>Gives my the following error and does not allow me to make the request at all:</p>
  
  <blockquote>
    <p>field "page_title" of type "[TextField]" must have a selection of
    subfields. Did you mean "page_title { ... }"?</p>
  </blockquote>
  
  <p>The error suggests that I need to add the nested subfield of <code>text</code> which is intended but when I use this query instead of the hardcoded one on the server it gives me the error mentioned before.</p>
  
  <p>Not sure if i've gone wrong somewhere in my setup?</p>
  
  <p>Thanks</p>