updateOne with nil value

I create a person object with an avatar (via swift) with following schema definition:

class Person : Object {
  @objc dynamic var avatar : Data?

if i dont set the avatar value it creates an entry with nil value

then when i like to update another field from the person via realm function it produce following error:

avatar: Invalid type. Expected: undefined, given: null

how can I add a Person object without setting avatar to nil? (I dont set it really to nil, it creates that automatically)