Got '&errors.errorString{s:"child url is nil"}' when expanding spec. about spec HOT 2 CLOSED

package main

import (
    "testing"

    "github.com/go-openapi/loads"
    "github.com/stretchr/testify/assert"
)

func TestSpec(t *testing.T) {
    swagger, err := loads.Spec("/tmp/swagger/api_v2.0.json")
    assert.Nil(t, err)

    swaggerExpanded, err := swagger.Expanded()
    assert.Nil(t, err)
}

type ExpandOptions struct {
    RelativeBase string
}

package main

import (
    "testing"

    "github.com/go-openapi/loads"
    "github.com/stretchr/testify/assert"
)

func TestSpec(t *testing.T) {
    swagger, err := loads.Spec("/tmp/swagger/api_v2.0.json")
    assert.Nil(t, err)

    swaggerExpanded, err := swagger.Expanded(&spec.ExpandOptions{
        RelativeBase: "/tmp/swagger",
    })
    assert.Nil(t, err)
}