Calling GetDocument results in "Error while copying content to stream"

I'm using the C# SDK from the GitHub repo and I'm trying to retrieve a signed PDF, but when I call the GetDocument method in the Agreements API it results in an error: "Error while copying content to stream". 

If I try the same call from Postman, I get an error ("read ECONRESET") response so it's almost as if the server is not allowing the method. Is there some trick to get this to work? Perhaps a header value that I'm not passing that I should be? Any help will be greatly appreciated.

This works:

Endpoint: https://api.na3.adobesign.com/api/rest/v6/agreements/[Agreement_ID_here]/documents

Headers:

    Authorization: Bearer [Integration_Key]

    Accept-Encoding: gzip, deflate, br
    Accept: */*,*_/_*;encoding=base64

    Connection: close (I have also tried keep-alive)

This does not work:

Endpoint: https://api.na3.adobesign.com/api/rest/v6/agreements/[Agreement_ID_here]/documents/[Document_ID_here]

Headers:

    Same as above