findByIdAndUpdate throwing cast error when I try and update a property to null

[Mongo Rookie]

Schema:

var SomeSchema = new Schema({

    name: { type: String, required: true, unique: true },

    description: { type: String, required: false }

  },{ 

    versionKey: false

  }

);

// In this case the client did not pass me a description, which is ok cause this property is not required.

// Why would the update fail?

var update = {name: someName, description: someDescription}; 

findByIdAndUpdate(id, update, function(err, something) { ...

Here is the error, yup cannot cast null/undefined to a String but why try?

CastError: Cast to string failed for value "undefined" at path "description"