用户如何在MS Access中插入null？

MS Access appears to support nulls in code, but I can't for the life of me figure out how to enter a null directly in a table. This is maddening because once a field has had a figure entered in it, it can never be deleted/set to null. Normally, allowing zero length strings would take care of this, but Access treats the XML export of a null and a zero length string differently. A null eliminates the associated XML tag and a zero length string sends an empty tag.MS Access appears to support nulls in code, but