python ValueError: I/O operation on closed file.

chunker=pd.read_csv("ch06\\ex2.csv",engine='python',chunksize=100)

tot=pd.Series([])

for piece in chunker:

    tot=tot.add(piece['key'].value_counts(),fill_value=0)

tot=tot.sort_value(ascending=False)

ValueError: I/O operation on closed file.