Parameters with multiple types fail validation

I'm seeing validation fail on Grape 0.15.0 for parameters which allow multiple types, as shown by the test case below. Replacing [Integer,String] with simply String or Integer works when provided the relevant format.

require 'grape'

class TestAPI < Grape::API
  params do
    # We originally saw the problem on a JSON parameter, so ensure it applies both on
    # that and a normal parameter.
    requires :message, type: JSON do
      requires :id, type: [Integer,String]
    end
    requires :id, type: [Integer,String]
  end
  post do
    status 200
    { message_id: params[:message][:id], id: params[:id] }
  end
end

run TestAPI

This request will return message[id] is invalid, id is invalid.

POST  HTTP/1.1
Host: localhost:9292
Content-Type: application/json
Cache-Control: no-cache

{
    "message": "{ \"id\": \"12\" }",
    "id": "12"
}

Start by reproducing the supplied Grape API example with both top-level and nested parameters, then trace the parameter validation path for type arrays. Done means values matching either Integer or String are accepted in both contexts without invalid-parameter errors.