IOException: "Received authentication challenge is null" (Apache Harmony/Android)

I am trying to send a GET via Android's HttpURLConnection (imported from org.apache.harmony.luni.internal.net.www.protocol.http.HttpURLConnection), and upon receiving the response, an IOException is thrown:

in doRequestInternal(): "Received authentication challenge is null"

What does this error mean, and what is causing this? I am writing OAuth parameters to the Authorization header, but I do this on other occasions, too, without problems.

    if (connection == null) {
        connection = (HttpURLConnection) new URL(endpointUrl).openConnection();
        connection.setRequestMethod("GET");
    }

    //... do some OAuth message signing

    connection.connect();

    int statusCode = connection.getResponseCode(); // throws IOException