how to handle public folder item using EWS 2010?

I continuously got the exception when calling GetItem upon public folder items in EWS 2010 as following,

System.Web.Services.Protocols.SoapException: The mailbox that was requested doesn't support the specified RequestServerVersion.

I'm using Impersonation and Exchange 2010, i'm pretty sure that i'm using the correct exchanegService version.  When trying to use version Exchange2007_SP1, another error message got:

System.Web.Services.Protocols.SoapException: An internal server error occurred. The operation failed.

The strange thing is it's all OK when doing this upon an item other folders(not public folder).

Anyone else happened to be stuck in the same problem? or anyone can share how to handle public folder item in EWS 2010?

thanks, and much appreciated!

Pi Jun Jiang