Unable to do NextU/PrevU on control points of objects in attached files

When I attach a file in a worksession, I can turn on the control points of e.g. a curve in that attached file. After selection of a control point, when doing NextU/PrevU Rhino tells me that at least one control point must be selected. If I copy the curve from the attached file into the current document, the NextU/PrevU works as expected.