--help throws TypeError: Cannot read property 'replace' of undefined

[springcomp]

I’m learning to author Yeoman Generators and would like to use TypeScript.
I stumble across the following issue which has been closed. #1717

Here is a repro case:
https://github.com/springcomp/yeoman-custom-generator

The generator and its subcommand work (just prompting and logging for now).
However displaying help fails:

yo sample --help
yo sample:router --help

With the following message:

Error sample --help 
Cannot read property 'replace' of undefined

I cannot figure out how to troubleshoot this.