I believe there is a small type-o in the php\Datatables\Query.php file in the recent 1.6.5 version. The public where_group function has an is_object($key) check where $key is undefined on line 661.
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You are absolutely correct - thank you for pointing that out!
It should be:
It will be in the next version (be that 1.6.6 or 1.7 - I'm not sure yet).
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