Can you try to apply [this patch](https://gist.github.com/stof/7157964) in your Symfony XmlEncoder to see if the warning disappears.
It should now throw the same exception than when detecting other decoding failures, which can then be catched by FOSRestBundle in case the behavior of a failed decoding is not an exception (I think it should return ``null`` in case of failure to be consistent with the json decoder)