Cannot use types.custom as part of a types.union

[univerio]

As demonstrated by the following example, changing a types.union(types.null, types.custom) field from null to a value of the custom type silently fails.

import {types} from "mobx-state-tree";

const customString = types.custom<string, string>({
  name: "custstring",
  fromSnapshot(s: string) {
    return s;
  },
  toSnapshot(s: string) {
    return s;
  },
  isTargetType(v: any) {
    return typeof v === "string";
  },
  getValidationMessage(s: any) {
    if (typeof s !== "string") {
      return "Invalid string";
    }
    return "";
  },
});

const Foo = types.model({
  foo: types.union(types.null, customString),
  bar: types.union(types.null, types.string),
}).actions((self) => ({
  set(s: string | null) {
    self.foo = s;
    self.bar = s;
  },
}));

let foo = Foo.create({foo: null, bar: null});
console.log(foo.foo, foo.bar);  // null null
foo.set("foo");
console.log(foo.foo, foo.bar);  // null 'foo'  <---------------------

foo = Foo.create({foo: "foo", bar: "foo"});
console.log(foo.foo, foo.bar);  // 'foo' 'foo'
foo.set("bar");
console.log(foo.foo, foo.bar);  // 'bar' 'bar'
foo.set(null);
console.log(foo.foo, foo.bar);  // null null