Problem with FasterRCNN

I have implemented a FasterRCNN and I want to use it on COCO dataset but there is a problem inside the implementation of the PyTorch module. Whenever I pass to the model an image without objects (bboxes is an empy list) the model crashes when it tries to unpack the boxes coordinates.

The model I have used:

    model = torchvision.models.detection.fasterrcnn_resnet50_fpn(pretrained=True)
    in_features = model.roi_heads.box_predictor.cls_score.in_features
    model.roi_heads.box_predictor = FastRCNNPredictor(in_features, num_classes)

The lines of code with the error in torchvision/models/detection/ line 220:

    xmin, ymin, xmax, ymax = boxes.unbind(1)

    xmin = xmin * ratio_width
    xmax = xmax * ratio_width
    ymin = ymin * ratio_height
    ymax = ymax * ratio_height
    return torch.stack((xmin, ymin, xmax, ymax), dim=1)

here boxes is an empty list and so the unbind() fails to unpack. Someone can help me? For what I can see it seems to be a bad implementation of the module