FindFolders specific value has invalid HTTP Header Characters

Hi,  I recently started hitting a bug, and any suggestions on how to work around this would be greatly appreciated.

var ewsClient = new ExchangeService();
ewsClient.Url = new Uri("https://outlook.office365.com/EWS/Exchange.asmx");
ewsClient.Credentials = new OAuthCredentials(authResult.AccessToken);
ewsClient.ImpersonatedUserId =
new ImpersonatedUserId(ConnectingIdType.SmtpAddress, mailbox);

//Include x-anchormailbox header
ewsClient.HttpHeaders.Add("X-AnchorMailbox", mailbox);

var folders = ewsClient.FindFolders(WellKnownFolderName.Inbox, new FolderView(10));

This last line throws an error:

Specified value has invalid HTTP Header characters.
Parameter name: name

Dezhi Li-MSFT 

Amit Singh Sansoya