WebFacing Drop-Down Box

I used the example in the "Mastering the IBM WebFacing Tool" redbook to 
create a drop-down, this is the snippet of code from the redbook:

<SPAN class="wf_blue wf_field" <%if (isProtected) {%> <% } else { 
%>TITLE="Click to select
with Calendar" STYLE="cursor: pointer; cursor: hand;"
onClick="show_calendar('window.opener.document.SCREEN.l<%=zOrder%>_SCRN01$DOFMON',
'window.opener.document.SCREEN.l<%=zOrder%>_SCRN01$DOFDAY',
'window.opener.document.SCREEN.l<%=zOrder%>_SCRN01$DOFYR',
'window.opener.document.SCREEN.DepMon', 
'window.opener.document.SCREEN.DepDay',
'window.opener.document.SCREEN.DepYea', 'N')" 
ONMOUSEOVER="this.style.background='yellow';"
ONMOUSEOUT="this.style.background=document.body.currentStyle.backgroundColor;" 
<%}%>>Date
of Delivery:</SPAN>

Everything works fine, the calendar pops up, and it passes the value back 
to my main screen, however there is an error on the page once the date 
gets passed back:
Line: 43
Char: 1
Error: 'undefined' is null or not an object.
Code: 0
URL: http://blah/WebFacing.do

Does the code for the drop down look okay?

    Dave Reiher
    System Analyst
    Prairie Farms Dairy - Corporate
    Ph: 217.854.2547 ext. 254
    dreiher@xxxxxxxxxxxxxxxx