為什麼我的 SoftLayer Go SDK GET 請求無法擷取 ItemCategory 資料？

// Endpoint for RESTful API calls
endpoint := "https://api.softlayer.com/rest/v3"

// Create a new session with the REST endpoint
sess := session.New(username, apikey, endpoint)

// Get SoftLayer_Account service
service := services.GetProductPackageService(sess)

// Mask to get specific configuration properties
mask := "itemCategory"

// Call to retrieve configuration, including item category
result, err := service.Mask(mask).Id(257).GetConfiguration()
if err == nil {
    // ItemCategory data should now be available in the result object
}