Types when using reaction() with fireImmediately are wrong during the immediate fire

[iownbey]

import { observableBox, reaction } from "@fobx/core";

const a = observableBox("b");

reaction(
  () => a.value,
  (newValue, oldValue) => {
    console.log(oldValue);
  },
  {
    fireImmediately: true,
  }
);

in this code snippet, oldValue is marked as having a type of string, even though when it is run, it has a value of undefined.
For these types to be correct, the types must include undefined iff fireImmediately is true.