ValueError: a must be greater than 0 unless no samples are taken while pretraining using cli

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Hi,
Sorry if this a duplicate question.
I prepared a raw text data like the format spacy ask for.
mydata.jsonl

{"text": "ホッケーにはデンジャラスプレーの反則があるので、膝より上にボールを浮かすことは基本的に反則になるが、その例外の一つがこのスクープである。"}
{"text": "また行きたい、そんな気持ちにさせてくれるお店です。"}

and my pretraining cli command is:

python -m spacy pretrain mydata.jsonl ja_core_news_lg outpath

After running this command I got this error:
Please help. Thanks in advance.

:information_source: Using GPU
:warning: Output directory is not empty
It is better to use an empty directory or refer to a new output path, then the
new directory will be created for you.
:heavy_check_mark: Saved settings to config.json
:heavy_check_mark: Loaded input texts
:heavy_check_mark: Loaded model 'ja_core_news_lg'

============== Pre-training tok2vec layer - starting at epoch 0 ==============

# Words Total Loss Loss w/s

Traceback (most recent call last):
File "/home/nsl7/anaconda3/envs/spacygpu2.3/lib/python3.6/runpy.py", line 193, in _run_module_as_main
"main", mod_spec)
File "/home/nsl7/anaconda3/envs/spacygpu2.3/lib/python3.6/runpy.py", line 85, in _run_code
exec(code, run_globals)
File "/home/nsl7/anaconda3/envs/spacygpu2.3/lib/python3.6/site-packages/spacy/main.py", line 33, in
plac.call(commands[command], sys.argv[1:])
File "/home/nsl7/anaconda3/envs/spacygpu2.3/lib/python3.6/site-packages/plac_core.py", line 367, in call
cmd, result = parser.consume(arglist)
File "/home/nsl7/anaconda3/envs/spacygpu2.3/lib/python3.6/site-packages/plac_core.py", line 232, in consume
return cmd, self.func(*(args + varargs + extraopts), **kwargs)
File "/home/nsl7/anaconda3/envs/spacygpu2.3/lib/python3.6/site-packages/spacy/cli/pretrain.py", line 237, in pretrain
model, docs, optimizer, objective=loss_func, drop=dropout
File "/home/nsl7/anaconda3/envs/spacygpu2.3/lib/python3.6/site-packages/spacy/cli/pretrain.py", line 264, in make_update
predictions, backprop = model.begin_update(docs, drop=drop)
File "/home/nsl7/anaconda3/envs/spacygpu2.3/lib/python3.6/site-packages/spacy/_ml.py", line 837, in mlm_forward
mask, docs = _apply_mask(docs, random_words, mask_prob=mask_prob)
File "/home/nsl7/anaconda3/envs/spacygpu2.3/lib/python3.6/site-packages/spacy/_ml.py", line 884, in _apply_mask
word = _replace_word(token.text, random_words)
File "/home/nsl7/anaconda3/envs/spacygpu2.3/lib/python3.6/site-packages/spacy/_ml.py", line 904, in _replace_word
return random_words.next()
File "/home/nsl7/anaconda3/envs/spacygpu2.3/lib/python3.6/site-packages/spacy/_ml.py", line 865, in next
numpy.random.choice(len(self.words), 10000, p=self.probs)
File "mtrand.pyx", line 902, in numpy.random.mtrand.RandomState.choice
ValueError: a must be greater than 0 unless no samples are taken