problem with zigzagline

["Amy Brisben, Ph.D." <amy bris verizon net>]

When I try to "add segment" to a zigzagline, a warning window pops up:
"This object doesn't support Undo/Redo. Undo information erased." and a
segment doesn't add.  I have no problems with adding
segments/corners/connection points for other line/shape types w/ these
functionalities.  Please advise.