Version 1.9.1 has introduced a bug that crashes the middleware by trying to access headers with response.headers. This will produce an error at least on express 4.17.1. I believe that the proper way of accessing the headers would be via response.get.
Error produced:
TypeError: Cannot read property 'server-timing' of undefined
at ServerTiming.addHeaders (/home/phz/workspace/server-timing-header-bug/node_modules/server-timing-header/index.js:283:37)
Minimal example can be found at https://github.com/Hallian/server-timing-header-bug-minimal-example