CSharp SDK await dbx.Files.UploadAsync(...) object gets disposed before upload

Hi, although I've mastered uploading text content to Dropbox, when trying to upload byte content (typical file, 18 MB), I get the error:

"ERROR: The object was used after being disposed"

The code is as follows, and the error was thrown in a try-catch in the second code block:

 

using (var dbx = new DropboxClient("My_Token"))
{
 await UploadToDropBox(dbx, "/test", IPATargetFileName, fullPathToFileToUpload);
}
// the method called is in the next code block

async Task UploadToDropBox(DropboxClient dbx, string dropboxfolder, string dropboxfilename, string filepathtoUpload)
{
 using (var mem = new MemoryStream(File.ReadAllBytes(filepathtoUpload)))
 {
 try
 {
 var updated = await dbx.Files.UploadAsync(
 dropboxfolder + "/" + dropboxfilename,
 WriteMode.Overwrite.Instance,
 body: mem);
 }
 catch (Exception ex)
 {
 LogMessage(string.Format(" : ERROR: {0}", ex.Message));
 }
 }
}

I'm, guessing that this is a MemoryStream / FileStream problem but the API example only shows how to upload a text file.

Kind regards,

Anthony