Here’s a function to ensure a Null value gets inserted into the database.

Function CheckForNULL(strStringToCheck)
	if len(trim(strStringToCheck)) = 0 then
		CheckForNULL =dbNull.Value
	else
		CheckForNULL = strStringToCheck
	end if
end function

If you have a DateTime field, without a function like this, a default date will be entered (ie 1/1/1900 12:00:00 AM), instead of null, when your insert is executed.