about keras.utils.to_categorical(y, num_classes=self.n_classes)

[hktxt]

I have n_classes = 1251, but it come up with error:

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RemoteTraceback Traceback (most recent call last)
RemoteTraceback:
"""
Traceback (most recent call last):
File "/data/hktxt/anaconda3/envs/tensorflow/lib/python3.6/multiprocessing/pool.py", line 119, in worker
result = (True, func(*args, **kwds))
File "/data/hktxt/anaconda3/envs/tensorflow/lib/python3.6/site-packages/keras/utils/data_utils.py", line 401, in get_index
return _SHARED_SEQUENCES[uid][i]
File "", line 28, in getitem
X, y = self.__data_generation(list_IDs_temp)
File "", line 55, in __data_generation
return X, keras.utils.to_categorical(y, num_classes=self.n_classes)
File "/data/hktxt/anaconda3/envs/tensorflow/lib/python3.6/site-packages/keras/utils/np_utils.py", line 34, in to_categorical
categorical[np.arange(n), y] = 1
IndexError: index 1251 is out of bounds for axis 1 with size 1251