title: Formik - Nested field validation
body: <p>I have nested fields with number inputs and can't get the validation to work properly. Not sure if this is a problem with formik or yup / how the validation schema is declared but I'll start asking here.</p>
  
  <p>In the example I have two fields which represents numbers and defaults to empty string. The validation works for the first fields but I can not get it to behave the same for the nested field. When I touch the field but don't type anything it returns :</p>
  
  <blockquote>
    <p>social.facebook must be a <code>number</code> type, but the final value was:
    <code>NaN</code> (cast from the value <code>""</code>).</p>
  </blockquote>
  
  <p>Example: <a href="https://codesandbox.io/s/formik-nested-schema-example-q4dlj?fontsize=14" rel="noreferrer">codesandbox</a></p>