find_all() fails on a field which is not specified

[FPurchess]

Document

class MyDocument(Document):
    pass

Schema

{
    unspecified_field: {}
}

Traceback

[...]
  File "motor/__init__.py", line 629, in _callback
    callback(None, e)
  File "motorengine/queryset.py", line 462, in handle
    raise arguments[1]
KeyError: u'unspecified_field'

[FPurchess]

I'd have contributed a patch, but unfortunately it's really hard to debug because comments are missing :/