typescript issues when a fixture extends a fixture

hi all, i have a base fixture which configures some global bits like an authenticated user. in some cases i want to extend that base fixture to test some edge cases, e.g. an unautheticated user. consider the following

import { test as base } from '@playwright/test'

type BaseFixture = {
  user: { firstName: string; lastName: string }
}

const myFixture = base.extend<BaseFixture>({
  user: {
    firstName: 'Nick',
    lastName: 'Williams',
  },
})

i would then extend my fixture like so:

const myOtherFixture = myFixture.extend({
   // Type '{ firstName: string; lastName: string; }' is not assignable to type 'undefined'
  user: {
    firstName: 'John',
    lastName: 'Doe',
  }
})

the problem is that i get a confusing typescript error as shown above. after some trial and error i notice that i can make this error go away by explicitly passing the generic type parameter to extend:

const myOtherFixture = myFixture.extend<BaseFixture>({
  user: {
    firstName: 'John',
    lastName: 'Doe',
  }
})

am i doing something wrong to trigger the TS error? is this a bug in playwright? or is it expected that you always explicitly pass the type param (in which case, should it be made non-optional)?