Error with key 'mono'

Key mono in POSITIONER giving error due to some images not having metadata

Traceback (most recent call last):
  File "/home/esrf/garrigaf/Documents/git/darfix/darfix/gui/dimensionsWidget.py", line 266, in _find_dimensions
    self.dataset.find_dimensions(self.metadata_type, self.tolerance)
  File "/home/esrf/garrigaf/Documents/git/darfix/darfix/core/dataset.py", line 432, in find_dimensions
    values = numpy.array([self.get_metadata_values(kind=kind, key=key) for key in keys])
  File "/home/esrf/garrigaf/Documents/git/darfix/darfix/core/dataset.py", line 432, in 
    values = numpy.array([self.get_metadata_values(kind=kind, key=key) for key in keys])
  File "/home/esrf/garrigaf/Documents/git/darfix/darfix/core/dataset.py", line 453, in get_metadata_values
    return [data.get_value(kind=kind, name=key)[0] for data
  File "/home/esrf/garrigaf/Documents/git/darfix/darfix/core/dataset.py", line 453, in 
    return [data.get_value(kind=kind, name=key)[0] for data
  File "/home/esrf/garrigaf/Documents/environments/darfix/lib/python3.8/site-packages/silx/io/fabioh5.py", line 515, in get_value
    value = self.__get_dict(kind)[name]
KeyError: 'mono'