Sort Order With Null Values

I have a Form which presents the results from a query. Upon clicking the header of each column of data the records are sorted A-Z and Z-A using the code below. This works fine when all records have data in the relevant field but not when several records have a value of null. It appears that when null values are present the "If Me.OrderBy = "property" & " DESC"" always evaluates as not true, even though all the null values are on top and all the filled in values are in appropriate order.

A second issue is one of my fields is not sorting properly at all. I have two date fields, one sorts the other does not. The one that sorts is a user entered date (8/22/2012) which defaults to "9/9/9999" if the user does not enter a different value. The other date field which does not sort properly at all is a calculated date field which chooses the earlier of two possible dates (both associated with the same record) using

IIf([Senior Mortgage Maturity]>[Mezz Maturity],[Mezz Maturity],[Senior Mortgage Maturity])

This code is repeated for each of the six fields with the relevant field name in place of 'property'.

Private Sub Property_Label_DblClick(Cancel As Integer)
Me.OrderByOn = True
If Me.OrderBy Like "Property" Then
Me.OrderBy = "property" & " DESC"
Else
Me.OrderBy = "property"
End If
End Sub