How can I get the "Last user defined annotation plane "

取个什么名好

When creating a pmi dimension, we can define its annotation plane in many ways,one of them is "Last user defined annotation plane "
right now, I want to get "Last user defined annotation plane " in my program, using
Xform xform = Session.GetSession().Parts.Work.Annotations.GetDefaultAnnotationPlane(NXOpen.Annotations.PmiDefaultPlane.Last);
but exception occurs, the error message is "Input is wrong.".
if I change the parameter,
Xform xform = Session.GetSession().Parts.Work.Annotations.GetDefaultAnnotationPlane(NXOpen.Annotations.PmiDefaultPlane.XyOfWcs);
It works fine!
Maybe I should set the annotation plane first,but I don't know the method either.
Can anybody help?
Thanks in advance.