MenuItem.MenuItems.Clear NotSupportedException

When I call myMenuItem.MenuItems.Clear() during runtime, I get a

"An unhandled exception of type 'System.NotSupportedException' occurred in System.Windows.Forms.dll

Additional information: NotSupportedException"

This same error occurs if I call myMenuItem.MenuItems.Remove(menu_i) in a for loop at the point when myMenuItem.MenuItems.Count is down to 1. I can remove all menu items up until that point successfully.

I posted to forums.microsoft.com here.