Clarification on "'wrong stringp nil type' issue, gem root and .gemspec file"

[amake]

Can you clarify this issue in the TODO file? Are there reproduction steps?
https://github.com/ruby-test-mode/ruby-test-mode/blame/master/TODOs.org#L4

I actually have an inkling because I saw a similar error when testing #59. The problem was that nil can be unexpectedly passed to some of the directory-related predicate functions in some situations; I solved it for my case by testing for nil:
605947d#diff-76e10da3a7d4a3940f01875e62211373827eb99cac8ebc0fb5acbcf7093fe085R527