Error while running gzsd.py

[SodaCoder]

When I do:
python gzsd.py

I get the following error:

Traceback (most recent call last):
File "gzsd.py", line 59, in
model = retinanet.retinanet_bbox(inputs=inputs, num_classes=num_seen, backbone=resnet)
File "/data2/charades2/PL-ZSD_Release/keras_retinanet/models/retinanet_vocab_w2v.py", line 350, in retinanet_bbox
model = retinanet(inputs=inputs, num_classes=num_classes, **kwargs)
File "/data2/charades2/PL-ZSD_Release/keras_retinanet/models/retinanet_vocab_w2v.py", line 307, in retinanet
_, C3, C4, C5 = backbone.outputs # we ignore C2
ValueError: not enough values to unpack (expected 4, got 1)

Could you kindly resolve this issue?