def _validate_label_map(label_map):
    """Checks if a label map is valid.

    Args:
      label_map: StringIntLabelMap to validate.

    Raises:
      ValueError: if label map is invalid.
    """
    for item in label_map.item:
        if item.id < 0:
            raise ValueError('Label map ids should be >= 0.')
        if (item.id == 0 and item.name != 'background' and item.
            display_name != 'background'):
            raise ValueError(
                'Label map id 0 is reserved for the background label')
