Conditional fields on typescript interface

Question

I am trying to define a type where a field is mandatory only if there is another field (that is optional). I have tried a very simple case, but I can't understand why it doesn't work. I have noticed also that the same solution was suggested in this post.

interface A {
  address: string
  data?: string
}

interface B extends A {
  nonce: number
  callbackAddress?: string
}

export type Configuration = string | A | B

const conf: B = {
    address: '',
    callbackAddress: '',
    nonce: 1 // if I try to remove this field, I get an error.
}

// this is the complete interface
const conf2: Configuration = { address: '', nonce: 123, callbackAddress: '0x' }

// I expect 'nonce' to be required here
const conf1: Configuration = { address: '', callbackAddress: '0x' }

Why doesn't the last line raise an error?