vue.js – initial reactive value of origin to null

everybody.
I have a question about using reactive. I know the type of a state, but the initial value is empty. How do I use assignment? The code is as follows

//....
  let origin = reactive<Timeslot | {}>({});
  const handler = (data) => {
       Object.assign(origin, data);
  };
//..

In fact, I want to assign the initial value of origin to null. How can I deal with this problem?