crashed to flush np.object type carray

[rabbit8ge]

import pandas as pd, numpy as np
import bcolz

df = pd.DataFrame({'a':[1,2,3]},dtype=np.object)
ct = bcolz.ctable.fromdataframe(df,rootdir='e:/tmp')
ct.flush()
ct = bcolz.open('e:/tmp')
ct.flush() # crashed in this line.