ValueError: need at least one array to concatenate

在用mmdetection进行coco数据集训练的时候遇到的问题

Traceback (most recent call last):
  File "tools/train.py", line 161, in <module>
    main()
  File "tools/train.py", line 157, in main
    meta=meta)
  File "/mmdetection/mmdet/apis/train.py", line 179, in train_detector
    runner.run(data_loaders, cfg.workflow, cfg.total_epochs)
  File "/opt/conda/lib/python3.7/site-packages/mmcv/runner/runner.py", line 384, in run
    epoch_runner(data_loaders[i], **kwargs)
  File "/opt/conda/lib/python3.7/site-packages/mmcv/runner/runner.py", line 279, in train
    for i, data_batch in enumerate(data_loader):
  File "/opt/conda/lib/python3.7/site-packages/torch/utils/data/dataloader.py", line 279, in __iter__
    return _MultiProcessingDataLoaderIter(self)
  File "/opt/conda/lib/python3.7/site-packages/torch/utils/data/dataloader.py", line 746, in __init__
    self._try_put_index()
  File "/opt/conda/lib/python3.7/site-packages/torch/utils/data/dataloader.py", line 861, in _try_put_index
    index = self._next_index()
  File "/opt/conda/lib/python3.7/site-packages/torch/utils/data/dataloader.py", line 339, in _next_index
    return next(self._sampler_iter)  # may raise StopIteration
  File "/opt/conda/lib/python3.7/site-packages/torch/utils/data/sampler.py", line 200, in __iter__
    for idx in self.sampler:
  File "/mmdetection/mmdet/datasets/samplers/group_sampler.py", line 36, in __iter__
    indices = np.concatenate(indices)
  File "<__array_function__ internals>", line 6, in concatenate
ValueError: need at least one array to concatenate