robot framework: org.openqa.selenium.NoSuchSessionException

I'm trying to get my first robot test running, but whatever I try, I get the following exception:

org.openqa.selenium.NoSuchSessionException

My test looks like

*** Settings ***
Documentation  A resource file containing the application specific keywords
Library     Selenium2Library

*** Test Cases ***
Check out www google dot com
    Open Browser  https://www.google.com  htmlunit
    Close Browser

Im using the following versions:

    <version.robot>3.0.2</version.robot>
    <version.robot.maven.plugin>1.4.7</version.robot.maven.plugin>
    <version.selenium2library>1.4.0.8</version.selenium2library>