System.Security.Principal.WindowsIdentity.get_AuthenticationType() throws exception

Can anyone tell me what the minimum user rights are to successfully call this method each and every time, under IIS 6.0 Windows Authentication with identity impersonation on?
I'm getting one of the following errors recorded in my event log, at least once every day:
 System.Security.Principal.WindowsIdentity.get_AuthenticationType() at System.Web.HttpRequest.CalcDynamicServerVariable(DynamicServerVariable var) at System.Web.HttpServerVarsCollectionEntry.GetValue(HttpRequest request) at System.Web.HttpServerVarsCollection.GetServerVar(Object e) at System.Web.HttpServerVarsCollection.Get(Int32 index) at System.Web.HttpServerVarsCollection.GetValues(Int32 index) at System.Collections.Specialized.NameValueCollection.Add(NameValueCollection c) at System.Web.HttpRequest.FillInParamsCollection() at System.Web.HttpRequest.GetParams() at System.Web.HttpRequest.get_Params() at AjaxControlToolkit.Toolki ...