Saxon cannot find function: current-group

I am trying to use Saxon with XSLT stylesheets and using the code examples in the XSLT2 spec (http://ift.tt/1CBlZzh)

<table xsl:version="2.0" xmlns:xsl="http://ift.tt/tCZ8VR">
  <tr>
    <th>Position</th>
    <th>Country</th>
    <th>City List</th>
    <th>Population</th>
  </tr>
  <xsl:for-each-group select="cities/city" group-by="@country">
    <tr>
      <td><xsl:value-of select="position()"/></td>
      <td><xsl:value-of select="@country"/></td>
      <td>
        <xsl:value-of select="current-group()/@name" separator=", "/>
      </td>
      <td><xsl:value-of select="sum(current-group()/@pop)"/></td>
    </tr>
  </xsl:for-each-group>
</table>

I'm using the following in my pom.xml

<dependency>
            <groupId>net.sf.saxon</groupId>
            <artifactId>Saxon-HE</artifactId>
            <version>9.6.0-3</version>
        </dependency>   

and the code to run it is:

    @Test
    public void testSaxonXslt2GroupTest1() throws Exception {
//      http://ift.tt/1CBlZzk  

        File xml_file = Fixtures.XSLT2_TEST1_XML;
        File xsl_file = Fixtures.XSLT2_TEST1_XSL;


        TransformerFactory tfactory = net.sf.saxon.TransformerFactoryImpl.newInstance();
        Transformer transformer = tfactory.newTransformer(new StreamSource(xsl_file));
        File saxonDir = new File("target/saxon/");
        saxonDir.mkdirs();
        try {
            transformer.transform(new StreamSource(xml_file),  
                new StreamResult(new FileOutputStream(new File(saxonDir, "test1.xml"))));
        } catch (Throwable t) {
            t.printStackTrace();
        }
    }

This throws an error on the output console

SystemId Unknown; Line #13; Column #70; Could not find function: current-group
SystemId Unknown; Line #13; Column #70; function token not found.
(Location of error unknown)java.lang.NullPointerException

Is this function missing in the Saxon version I am using, or am I doing something wrong?