how to run dropwizard client-testing example? (jettyServer is null at DropwizardTestSupport.getLocalPort())

I'm trying to get the excample test from https://www.dropwizard.io/en/stable/manual/testing.html#testing-client-implementations to work. As stated below the code section "The DropwizardClientRule takes care of: […] Choosing a free random port number (important for running tests in parallel).". That does not seem to work. I'm getting a NullPointerException when using DropwizardClientRule, as jettyServer object is null at io.dropwizard.testing.DropwizardTestSupport.getLocalPort().

Am I missing something, or is this a bug?