Custom File Browser: CKEditorFuncNum=undefined

Hi!
I have multiple instances of Ckeditor on a single page with shared toolbar and I am using a custom File Browser (no ckfinder) to insert links. When selecting 'Browser Server', it opens correctly the desired page.

config.filebrowserBrowseUrl = './workspacebrowser.jsf';
config.filebrowserWindowWidth = '600';
config.filebrowserWindowHeight = '600';

After selecting the desired link, I would like to set the correct URL.

window.opener.CKEDITOR.tools.callFunction(funcNum, fileUrl);

I get the correct instance of the the current editor but the CKEditorFuncNum=undefined

http://localhost:8080/nymphaea/pages/workspacebrowser.jsf?CKEditor=j_id313:contentAreaDataTable:0:formcontent:content&CKEditorFuncNum=undefined&langCode=en

Can any one give an idea on how to get the value of CKEditorFuncNum or why is CKEditorFuncNum undefined is in my case?

Thanks,
Choesang