'strict-type-predicates' error occurs on `yarn test`

[Rindrics]

• I'm submitting a ...
  [ ] language request
  [x] bug report
  [ ] feature request
  [ ] question about the decisions made in the repository
  [ ] question about how to use this project

• Summary

I encountered the error below when I executed yarn test on 7da4097:

❯ yarn test
yarn run v1.22.19
$ yarn lint && yarn unit && nyc report --reporter=lcov && yarn check-coverage
$ tslint --project . src/**/*.ts
/Users/akirahayashi/dev/adr/src/lib/cache.ts:47:12
ERROR: 47:12  strict-type-predicates  Expression is always true.

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. StackOverflow, personal fork, etc.)

I avoided the error on my branch: https://github.com/Rindrics/adr/tree/avoid-strict-type-predicates-error