open-wc: [testing-karma] running karma with lerna on default config, results in an error

For some reason it does not happen with open-wc config

when I run

lerna exec karma start -- -c karma.conf.js

I get, which is not very helpful

01 08 2019 12:56:06.661:ERROR [karma-server]: UncaughtException

But when run with limited concurrency, it works

lerna exec --concurrency=1 karma start -- -c karma.conf.js

I have hunch, that it might be related to ports. Karma server by default tries to find open ports before starting server I do not see explicit setting of a port in open-wc config, but not sure why it does not work