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setting default image in dhtmlxcombo

I am attempting to change the default image used in dhtmlxcombo but the addition of dhtmlxcombo_extra.js to my script causes an error:
Error: dhtmlXCombo_defaultOption is not defined
Source File: http://qi-sales.com/common/javaScript/dhtmlX/dhtmlxCombo/codebase/ext/dhtmlxcombo_extra.js
Line: 11

The URL I am specifying is live: I have tested it, and I have specified the image using a fully qualified URI, a relative URI and an absolute URI.

I am using //v.1.3 build 81009, the latest version of dhtmlxcombo.

Please be sure that you have included both 
   dhtmlxcombo.js
   dhtmlxcombo_extra.js
and "extra" js included after main one.