Need to update activitypointer Entity

I need to update the "senton" date field which is available in activitypointer Entity.I use the following code for update the record but it shows the "The 'Update' method does not support entities of type 'activitypointer'." Error.

Entity FilteredActivityPointer = new Entity("activitypointer");

FilteredActivityPointer.Attributes["senton"] = DateTime.Now;
FilteredActivityPointer.Attributes["activityid"] = new Guid("<Record guid here>");
service.Update(FilteredActivityPointer);

How can I update the senton date filed in activitypointer ?