Set extended properties on calendar folder?

Hi,

I'm working on a synchronization application using EWS Managed API. Actually, I'm able to set extended properties on calendar folder (PublicStrings) and retrieve them in my application. But not in my Outlook addin, developed with VSTO. I'm doing the following :

MAPIFolder calendar = this.Application.Session.GetDefaultFolder(Outlook.OlDefaultFolders.olFolderCalendar) as MAPIFolder;
Object o = calendar.PropertyAccessor.GetProperty("http://schemas.microsoft.com/mapi/string/{00020329-0000-0000-C000-000000000046}/EmployeeTasks");

It doesn't work => COMException... But it works when I do it on appointmentItem. Answer? Thanks by advance!