TypeScript object definition with any typed key

How to create a swagger definition of an object with any keys, which all are of a defined type?

What I have

definitions:
  Language:
    type: object
    properties:
      foo:
        type: object
        items:
          $ref: "#/definitions/Foo"
  Foo:
    type: object
    additionalProperties:
      $ref: "#definitions/Bar"
  Bar:
    type: object
    properties:
      a:
        type: string
      b:
        type: number

What I want

export interface Language {
    foo?: Foo
}

export interface Foo {
    [key: string]: Bar
}

export interface Bar{
    a?: string
    b?: number;
}

What I currently receive

export interface Language {
    foo?: any
}
export interface Foo extends null<String, any> {
}

export interface Bar{
    a?: string
    b?: number;
}

No source files, tests, or entry points are named. Start by tracing the TypeScript generator output for the shown additionalProperties definition and compare it with the requested index signature; done means Foo uses string keys mapped to Bar and Language.foo is typed as Foo rather than any.