You have to pass true to nullable -

nrOfApples: yup.number().min(0).max(999).nullable(true);

From: https://github.com/jquense/yup/issues/500

Working example: https://runkit.com/xdumaine/5f2816c75a0ba5001aa312b2

Note that if you add required().nullable(true) the required overrides the nullable and null will not validate.

Update:

You can use a transform to convert the NaN value into null. I updated the runkit with this:

const contactSchema = yup.object({

  name: yup.string()

    .required(),

  nrOfApples: yup

    .number()

    .min(0)

    .max(999)

    .nullable(true)

    // checking self-equality works for NaN, transforming it to null

    .transform((_, val) => val === Number(val) ? val : null) 

})