WireMock stub creation fails: expects 201 but get 200

I am new to WireMock and trying to get my first unit test to work with it. Now, following the docs on wiremock.org, I wrote this

    WireMockConfiguration config = wireMockConfig().port(9089).httpsPort(8443);
    m_wireMockServer    =       new WireMockServer(config);

    m_wireMockServer.start();
    WireMock.configureFor("localhost", 9089);
    givenThat(get(urlEqualTo("/some/thing"))
    .willReturn(aResponse()
        .withHeader("Content-Type", "text/plain")
        .withBody("Hello world!")));

I expects this to make any http request to /some/thing to be caught. It gives me the following exception on the giventhat call:

com.github.tomakehurst.wiremock.client.VerificationException: Expected status 201 for http://localhost:9089/__admin/mappings/new but was 200
at com.github.tomakehurst.wiremock.client.HttpAdminClient.postJsonAssertOkAndReturnBody(HttpAdminClient.java:151)
at com.github.tomakehurst.wiremock.client.HttpAdminClient.addStubMapping(HttpAdminClient.java:65)
at com.github.tomakehurst.wiremock.client.WireMock.register(WireMock.java:130)
at com.github.tomakehurst.wiremock.client.WireMock.register(WireMock.java:126)
at com.github.tomakehurst.wiremock.client.WireMock.givenThat(WireMock.java:65)

What did I miss? What is wrong in the creation of the stub?