I am changing username (email which is the same) of a IMember programatically in my umbraco app.

            if(model.Email != member.Email)
            {
                member.Email = model.Email;
                member.Username = model.UsernameIsEmail ? model.Email : model.Username;
                updateLogin = true;
            }

After I save the member's new username and email I cannot longer access member only pages and getting the following error:

[NullReferenceException: Object reference not set to an instance of an object.]
   Umbraco.Web.Security.MembershipHelper.IsMemberAuthorized(Boolean allowAll, IEnumerable`1 allowTypes, IEnumerable`1 allowGroups, IEnumerable`1 allowMembers) +136
   Umbraco.Web.Security.WebSecurity.IsMemberAuthorized(Boolean allowAll, IEnumerable`1 allowTypes, IEnumerable`1 allowGroups, IEnumerable`1 allowMembers) +145
   Umbraco.Web.Mvc.MemberAuthorizeAttribute.AuthorizeCore(HttpContextBase httpContext) +385
   System.Web.Mvc.AuthorizeAttribute.OnAuthorization(AuthorizationContext filterContext) +160
   System.Web.Mvc.ControllerActionInvoker.InvokeAuthorizationFilters(ControllerContext controllerContext, IList`1 filters, ActionDescriptor actionDescriptor) +97
   System.Web.Mvc.Async.<>c__DisplayClass21.<BeginInvokeAction>b__19(AsyncCallback asyncCallback, Object asyncState) +743
   System.Web.Mvc.Async.WrappedAsyncResult`1.CallBeginDelegate(AsyncCallback callback, Object callbackState) +14
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.Begin(AsyncCallback callback, Object state, Int32 timeout) +128
...

How to re-authenticate logged user after username update?

I tried recall

            if(updateLogin)
            {
                _membershipHelper.Login(model.Username, model.Password);
            }

but it didn't help.

Thanks