typescript-fetch: unexpected behavior when creating "any" type

I want to generate a model with a property that has the "any" type.
According to the OAS 3 spec, "Any Type" can be achieved by omitting the type property. See the section "Any Type" of https://swagger.io/docs/specification/data-models/data-types/
But typescript-fetch produces some unexpected results.

This behavior can be reproduced by modifying the Pet Store example on https://editor.swagger.io and generating the typescript-fetch client. The Pet Store example was converted to OAS 3 first.

For example:

    Pet:
      type: object
      properties:
        value:
          description: any value

When generating the typescript-fetch client the result is the following:

export interface Pet {
    /**
     * any value
     * @type {ModelObject}
     * @memberof Pet
     */
    value?: ModelObject;
}

The type ModelObject does not exist and the client can't be used that way.

After some tests I found out, that by adding additionalProperties, the result changes.

Adding additionalProperties: true results in:

export interface Pet {
    /**
     * any value
     * @type {{ [key: string]: any; }}
     * @memberof Pet
     */
    value?: { [key: string]: any; };
}

Adding additionalProperties: false results in:

export interface Pet {
    /**
     * any value
     * @type {any}
     * @memberof Pet
     */
    value?: any;
}

Please correct me if I am wrong, but I thought when additionalProperties is omitted, it is treated as if it was present with the value true.

Some additional info: Having a property with any type, the TSOA framework generates the following OAS:

"value": {
  "additionalProperties": true
}

Suggestion: If the type is omitted, typescript-fetch should create a property of type any. additionalProperties shouldn't influence the output if no type is provided.

Reproduce the issue by modifying the OAS 3 Pet Store example in editor.swagger.io and generating the typescript-fetch client. Compare output for an omitted type with the cases using additionalProperties true or false. Done means an omitted type generates a usable TypeScript property of type any without an undefined ModelObject reference.