How to get response header in c# Microsoft graph api request

Question

I am trying to do File Copy operation in c# .net core using Microsoft graph API. It is an asynchronous operation, and by doc, it says it returns a location in the response header to check the status of the operation, Now the issue is I need its response header so that I can check the status of file copy operation but every time I am getting 'null' as value, I have tried following code,

DriveItem response = await graphClient.Sites[siteId].Drive.Items[itemId]
                           .Copy(fileName, parentReference)
                           .Request()
                           .PostAsync();

The driveItem returns null but I think at least it should have returned the additional data-carrying response status and location.

When I use online graph api it just works fine returning response and location, but it doesn't with graph client service.