title: Why does a GraphQL query return null?
body: <p>I have an <code>graphql</code>/<code>apollo-server</code>/<code>graphql-yoga</code> endpoint. This endpoint exposes data returned from a database (or a REST endpoint or some other service).</p>
  
  <p>I know my data source is returning the correct data -- if I log the result of the call to the data source inside my resolver, I can see the data being returned. However, my GraphQL field(s) always resolve to null.</p>
  
  <p>If I make the field non-null, I see the following error inside the <code>errors</code> array in the response:</p>
  
  <blockquote>
    <p>Cannot return null for non-nullable field</p>
  </blockquote>
  
  <p>Why is GraphQL not returning the data?</p>