Error parsing hng6

[Keith-Hon]

Describe the bug
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Error thrown when calling pycantonese.parse_jyutping('hng6')

To reproduce
Steps to reproduce the behavior, including the complete stack trace if possible:
pycantonese.parse_jyutping('hng6')

Expected behavior
A clear and concise description of what you expected to happen instead.
Should return [Jyutping(onset='h', nucleus='ng', coda='', tone='6')]

Screenshots
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System (please complete the following information):

• Operating System: [e.g. Windows, MacOS, Linux]
• PyCantonese version: [What you see after you run import pycantonese; print(pycantonese.__version__)]

Additional context
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[Keith-Hon]

https://github.com/Keith-Hon/pycantonese/commit/241b5215feb29651cef107321485233b0d3fa63e

[yuzuda283]

same error on version 3.4