Cannot find extension function using 10.x

Hi,

I've been trying to upgrade to Saxon 10.x and had to implement a few
(simple) extension functions in the process. I've been following this
guide:
https://www.saxonica.com/html/documentation/extensibility/integratedfunctions/ext-simple-J.html

However I get this error message when attempting to use one of them:

Error near {...c:forClass, $ldt:base)/rdf:...} at char 0 in
xsl:apply-templates/@select on line 650 column 135 of layout.xsl:
  XPST0017: Cannot find a 3-argument function named
  Q{https://w3id.org/atomgraph/client#}construct-doc()

I've double-checked the namespaces.

The function is registered on the Processor:

        processor.registerExtensionFunction(new ConstructDocument(processor));

The function itself looks like this:

public class ConstructDocument implements ExtensionFunction
{

    private final Processor processor;

    public ConstructDocument(Processor processor)
    {
        this.processor = processor;
    }

    @Override
    public QName getName()
    {
        return new QName("https://w3id.org/atomgraph/client#", "construct-doc");
   }

    @Override
    public SequenceType getResultType()
    {
        return SequenceType.makeSequenceType(ItemType.DOCUMENT_NODE,
OccurrenceIndicator.ZERO_OR_MORE);
    }

    @Override
    public SequenceType[] getArgumentTypes()
    {
        return new SequenceType[]
        {
            SequenceType.makeSequenceType(ItemType.ANY_URI,
OccurrenceIndicator.ONE),
            SequenceType.makeSequenceType(ItemType.ANY_URI,
OccurrenceIndicator.ZERO_OR_MORE),
            SequenceType.makeSequenceType(ItemType.ANY_URI,
OccurrenceIndicator.ONE),
        };
    }

    @Override
    public XdmValue call(XdmValue[] arguments) throws SaxonApiException
    {
        ...
    }

    public Processor getProcessor()
    {
        return processor;
    }

}

What am I missing here? Thanks.