Conditional Validation in Yup : <p>I have an email field that only gets shown if a checkbox is selected (boolean value is <code>true</code>). When the form get submitted, I only what this field to be required if the checkbox is checked (boolean is true).  </p>

<p>This is what I've tried so far:</p>

<pre><code>    const validationSchema = yup.object().shape({
       email: yup
             .string()
             .email()
             .label('Email')
             .when('showEmail', {
                 is: true,
                 then: yup.string().required('Must enter email address'),
             }),
        })
</code></pre>

<p>I've tried several other variations, but I get errors from Formik and Yup:</p>

<p><code>Uncaught (in promise) TypeError: Cannot read property 'length' of undefined
    at yupToFormErrors (formik.es6.js:6198)
    at formik.es6.js:5933
    at &lt;anonymous&gt;
yupToFormErrors @ formik.es6.js:6198
</code></p>

<p>And I get validation errors from Yup as well. What am I doing wrong? </p>
