ImageCaptioning.pytorch: Eval runs ok first time but then throw an "ZeroDivisionError: division by zero"

hi

I got the eval code to run ok the first time on an image, but then when I try to run it again on the same image or on any other image I get the following error message. Is there some buffer that I need to clean up somewhere between each inference run?

<font color="#729FCF">ImageCaptioning.pytorch</font>$ sudo python3 eval.py --model models/fc/model-best.pth --infos_path models/fc/infos_fc-best.pkl --image_folder images/ --num_images 1
Hugginface transformers not installed; please visit https://github.com/huggingface/transformers
meshed-memory-transformer not installed; please run `pip install git+https://github.com/ruotianluo/meshed-memory-transformer.git`
Warning: coco-caption not available
loading annotations into memory...
Done (t=0.78s)
creating index...
index created!
Traceback (most recent call last):
  File "eval.py", line 72, in <module>
    lang_stats = eval_utils.language_eval(opt.input_json, predictions, n_predictions, vars(opt), opt.split)
  File "/media/tetsfr/SSD/ImageCaptioning.pytorch/captioning/utils/eval_utils.py", line 79, in language_eval
    mean_perplexity = sum([_['perplexity'] for _ in preds_filt]) / len(preds_filt)
ZeroDivisionError: division by zero

Same pattern if I change the pre-trained model used for evaluation, it works the first time but then throws this division by zero error all the time. thanks for your help.