Flow union functions?

error.log

result.js:17
 17:     callback(null, {value: Math.random()});
                  ^^^^ null. This type is incompatible with the expected param type of
 14:   callback : ResultCallback<V, E>
                                    ^ Error

result.js:19
 19:     callback(new Error('Lolnope'));
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ function call
 19:     callback(new Error('Lolnope'));
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ undefined (too few arguments, expected default/rest parameters). This type is incompatible with
 14:   callback : ResultCallback<V, E>
                                 ^ object type

result.js:19
 19:     callback(new Error('Lolnope'));
                  ^^^^^^^^^^^^^^^^^^^^ Error. This type is incompatible with the expected param type of
  3: type Callback<T> = (error: null, value: T) => void;
                                ^^^^ null

result.js:24
 24: const a = doSomething(true, (error, value) => {
                                 ^ arrow function. Could not decide which case to select
 14:   callback : ResultCallback<V, E>
                  ^^^^^^^^^^^^^^^^^^^^ union type
  Case 1 may work:
    6:   | Callback<R>
           ^^^^^^^^^^^ type application of polymorphic type: type `Callback`
  But if it doesn't, case 2 looks promising too:
    7:   | Errback<E>;
           ^^^^^^^^^^ type application of polymorphic type: type `Errback`
  Please provide additional annotation(s) to determine whether case 1 works (or consider merging it with case 2):
   24: const a = doSomething(true, (error, value) => {
                                    ^^^^^ parameter `error`
   24: const a = doSomething(true, (error, value) => {
                                           ^^^^^ parameter `value`

result.js:26
 26:     return errror;
                ^^^^^^ identifier `errror`. Could not resolve name


Found 5 errors
  

result.js

/** @flow */

type Callback<T> = (error: null, value: T) => void;
type Errback<T> = (error: T, value?: any) => void;
type ResultCallback<R, E> =
  | Callback<R>
  | Errback<E>;

type V = {value: number};
type E = Error;

const doSomething = (
  thing : boolean,
  callback : ResultCallback<V, E>
) => {
  if (thing) {
    callback(null, {value: Math.random()});
  } else {
    callback(new Error('Lolnope'));
  }
};


const a = doSomething(true, (error, value) => {
  if (error) {
    return errror;
  }
  // ideally, flow would know that value is of type `V` here because error is
  // null, though currently this is a type error 
  console.log(value);
});