Creating a Schema with Explicit Undefined in Type Signature

type MySchema = Schema.Schema<
  { a?: string | undefined; b?: string | undefined },
  { a?: string; b: Option<string> }
>;

const MySchema = Schema.struct({
  a: Schema.optional(Schema.string),
  b: Schema.optional(Schema.string).toOption(),
});

Schema.Schema<{
    readonly a?: string;
    readonly b?: string;
}, {
  readonly a?: string;
  readonly b: Option<string>;
}>

const decoded = decode({ a: undefined });
//                      ^? Type 'undefined' is not assignable to type 'string'.

const MySchema = Schema.struct({
  a: Schema.optional(Schema.union(Schema.string, Schema.undefined)),
  b: Schema.optional(Schema.union(Schema.string, Schema.undefined)).toOption(),
});