404-page uses content of optional archive sidebar if available

Hello, I noticed that the 404-page normally uses the default (primary) sidebar - which I really like. Recently I started to set up the optional sidebars for posts and archive (Dashboard -> widgets). To my surprise I discovered today that the 404-page now displays the content of the optional archive sidebar. Is this a mistake? (I've tested that it was really the sidebar for archive and not the one for posts). To be honest, I would rather like the default (primary) sidebar to be displayed at the 404-page at all times. How can I achieve this? Best regards, Henriët