CB_Handler error while running Resnet34 error

Tried running Resnet34 model, but got the below error.

Databunch was created as follows:

data = (
.label_from_func(lambda path: fn2label[path.name])
.transform(get_transforms(do_flip=False, max_zoom=1, max_warp=0, max_rotate=25), size=SZ, resize_method=ResizeMethod.SQUISH)
.databunch(bs=BS, num_workers=NUM_WORKERS, path=path)

This is a multi-class problem. Some of the images are only 1, is it due to image class present in train but not in validate dataset

Can someone please help!!