CKEditor throws javascript error on load

Hi

I am trying to use the ckeditor_source.js instead of of ckeditor.js as i need to play with plugins. Everything works 100% when using 'ckeditor.js' but when i try use the 'ckeditor_source.js' then the editor breaks. The problem is 'CKEDITOR.env' is null/undefined. 'env.js' is added to the 'scriptsloaded' attribute of CKEDITOR but the file is never actually loaded. Am i doing something incorrectly or is something wrong with the loading function?

Thanks.
Ty