Cannot find name 'ref'

I have an OpenAPI json like that:

{
  "swagger": "2.0",
  "definitions": {
    "Messages": {
      "type": "object",
      "additionalProperties": {
        "$ref": "#/definitions/Message"
      }
    },
    "Message": {
      "type": "object",
      "properties": {
        "code": {
          "type": "integer"
        },
        "text": {
          "type": "string"
        }
      }
    }
  }
}

Which results in typescript:

export interface definitions {
  Messages: { [key: string]: ref }
  Message: { code?: number; text?: string }
}

It is not valid, because of ref. What is ref, and why it is not defined?