TypeError: write() argument must be str, not None

[dclong]

I run the following command

format-sql a_sql_file.sql

but got the following error

Traceback (most recent call last):
  File "/usr/local/bin/format-sql", line 11, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.5/dist-packages/format_sql/main.py", line 83, in main
    _write_back(filename, lines, args.dry_run)
  File "/usr/local/lib/python3.5/dist-packages/format_sql/main.py", line 148, in _write_back
    f.write(lines)
TypeError: write() argument must be str, not None