Typescript can't assign value to object key with hasOwnProperty type narrowing

I defined hasOwnProperty with type narrowing:

function hasOwnProperty<
  Obj extends Record<string, any>,
  Prop extends PropertyKey,
>(
  obj: Obj,
  prop: Prop,
): obj is Obj & Record<Prop, any> {
  return Object.prototype.hasOwnProperty.call(obj, prop);
}

I expected a straightforward function like this to work:

function addProp<T>(obj: Record<string, any>, key: string, val: any) {
  if (!hasOwnProperty(obj, key)) {
    obj[key] = val;
  }
}

However, I'm getting Type 'any' is not assignable to type 'never'.

TS Playground

This kind of makes sense because the type narrowing is saying that key doesn't exist in obj. However, this shouldn't prevent me from adding the key. Is there something I'm doing wrong?

Edit: Also, it doesn't work outside of a function either:

const obj: Record<string, number> = {};
const key = 'key';
if (!hasOwnProperty(obj, key)) {
  obj[key] = 123;
}

from Typescript can't assign value to object key with hasOwnProperty type narrowing