Simple XML @ElementMap unable to satisfy key attribute

I'm trying to deserialise with Simple XML 2.6.2 using the @ElementMap annotation to build a map containing an element's attribute as the key and the element itself as the value.

The XML looks like this:

<ProcessConfiguration id="4020">
  <EquipmentConfigurations>
    <EquipmentConfiguration id="5020">
      <address>foo</address>
    </EquipmentConfiguration>
  </EquipmentConfigurations>
</ProcessConfiguration>

The annotated classes look like this:

@Root
class ProcessConfiguration {

  @Attribute
  Long id;

  @ElementMap(name = "EquipmentConfigurations", key="id", attribute = true)
  Map<Long, EquipmentConfiguration> equipmentConfigurations = new HashMap<>();
}

EquipmentConfiguration.java

@Root
class EquipmentConfiguration {

  @Attribute
  Long id;

  @Element
  String address;
}

As you can see, the equipmentConfigurations map should contain the EquipmentConfiguration ID as the map key, and the EquipmentConfiguration as the map value.

But when I try to deserialise, the following error is thrown:

Exception in thread "main" org.simpleframework.xml.core.ValueRequiredException: Unable to satisfy @org.simpleframework.xml.Attribute(required=true, empty=, name=) on field 'id' java.lang.Long EquipmentConfiguration.id for class EquipmentConfiguration at line 1

I've tried various things with the @ElementMap annotation, but without success.

I'm pretty stuck here, as I don't see how Simple isn't finding the id attribute. Are there any Simple wizards out there who can help?

Thanks in advance!