How do I dispose my filestream when implementing a file download in ASP.NET?

I have a class DocumentGenerator which wraps a MemoryStream. So I have implemented IDisposable on the class.

I can't see how/where I can possibly dispose it though.

This is my current code, which performs a file download in MVC:

using (DocumentGenerator dg = DocumentGenerator.OpenTemplate(path))
{
    /* some document manipulation with the 
       DocumentGenerator goes here ...*/

    return File(dg.GetDocumentStream(), "text/plain", filename);
}

This errors as the stream is closed/disposed before the controller has finished with it. How can I make sure my resources are properly disposed in this situation?

EDIT: My implementation of IDisposable at the moment just disposes the MemoryStream. I know it's not a proper implementation, I just used it as a test. Is there something different I could do here to make it work?

public void Dispose()
{
    _ms.Dispose();
    _ms = null;
}