Cannot use FaceCrop with a video sequence where all frames do not have annotations

I have a video file with the missing annotations on some frames:

{
 "0": {},
 "1": {},
 "2": {},
 "3": {},
 "4": {},
 "5": {},
 "6": {},
 "7": {},
 "8": {},
 "9": {},
 "10": {},
 "11": {},
 "12": {},
 "13": {},
 "14": {},
 "15": {
  "reye": [
   635,
   239
  ],
  "leye": [
   618,
   647
  ],
 }

and I want to use it FaceCrop wrapped in bob.bio.video with this video file. However it fails with:

bob.bio.base@2018-02-14 18:21:41,462 -- ERROR: During the execution, an exception was raised: At least one of the expected annotations '['leye', 'reye']' are not given in 'dict_keys([])'.
Traceback (most recent call last):
  File "bin/verify.py", line 77, in <module>
    sys.exit(bob.bio.base.script.verify.main())
  File "src/bob.bio.base/bob/bio/base/script/verify.py", line 435, in main
    verify(args, command_line_parameters)
  File "src/bob.bio.base/bob/bio/base/script/verify.py", line 415, in verify
    if not execute(args):
  File "src/bob.bio.base/bob/bio/base/script/verify.py", line 264, in execute
    force = args.force)
  File "src/bob.bio.base/bob/bio/base/tools/preprocessor.py", line 81, in preprocess
    preprocessed_data = preprocessor(data, annotations)
  File "src/bob.bio.video/bob/bio/video/preprocessor/Wrapper.py", line 126, in __call__
    preprocessed = self.preprocessor(frame, annots)
  File "src/bob.bio.face/bob/bio/face/preprocessor/FaceCrop.py", line 208, in __call__
    image = self.crop_face(image, annotations)
  File "src/bob.bio.face/bob/bio/face/preprocessor/FaceCrop.py", line 154, in crop_face
    raise ValueError("At least one of the expected annotations '%s' are not given in '%s'." % (self.cropped_keys, annotations.keys()))
ValueError: At least one of the expected annotations '['leye', 'reye']' are not given in 'dict_keys([])'.

What is the best strategy to avoid this error? I want the preprocessing to continue and drop those frames.