Przekształcenie mapy na obiekt xml

kiedy dodam adnotacje @XmlElementWrapper do moje POJO dostaję wyjątek Exception :

Exception in thread "main" javax.xml.bind.JAXBException: 
Exception Description: XmlElementWrapper is only allowed on a collection or array property but [movies] is not a collection or array property.
 - with linked exception:
[Exception [EclipseLink-50015] (Eclipse Persistence Services - @VERSION@.@QUALIFIER@): org.eclipse.persistence.exceptions.JAXBException
Exception Description: XmlElementWrapper is only allowed on a collection or array property but [movies] is not a collection or array property.]
    at org.eclipse.persistence.jaxb.JAXBContext$TypeMappingInfoInput.createContextState(JAXBContext.java:1068)
    at org.eclipse.persistence.jaxb.JAXBContext.<init>(JAXBContext.java:182)

Na to chwilę nie jestem pewien czy potrzebuję tej adnotacji.
Za jaką kolwiek pomoc będę strasznie wdzięczny!!!
Na chwilę obecną dostaję taki kod wynikowy :

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<issue>
    <custom_fields>
        <item id="1">
            <value>213</value>
        </item>
        <item id="2">
            <value>4356</value>
        </item>
    </custom_fields>
    <description>adam</description>
    <journals/>
    <subject>Subject</subject>
</issue>

a chciałbym otrzymać :

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<issue>
    <custom_fields>
        <custom_field id="1">
            <value>213</value>
        </custom_field>
        <custom_field id="2">
            <value>4356</value>
        </custom_field>
    </custom_fields>
    <description>adam</description>
    <journals/>
    <subject>Subject</subject>
</issue>

Moje klasy:
Issue.xml

@XmlRootElement(name = "issue")
public class Issue {

    @Expose
    private Integer id;
    private TypAplikacji project_id;
    private TypIncydentu tracker_id;
    @Expose
    private StatusZgloszenia status;
    private Author author;
    private String subject;
    private String description;

   private Map<CustomFields, String> customFields;
    @XmlElement(name = "custom_fields")
    @XmlJavaTypeAdapter(MapAdapter.class)
    public Map<CustomFields, String> getCustomFields() {
        return customFields;
    }

    public void setCustomFields(Map<CustomFields, String> customFields) {
        this.customFields = customFields;
        }
}

MapElements.java :

@XmlRootElement(name = "custom_field")
public class MapElements {

    private CustomFields key;
    private String value;

    public MapElements() {
    }

    public MapElements(CustomFields key, String value) {
        this.key = key;
        this.value = value;
    }

    @XmlAttribute(name = "id")
    public CustomFields getKey() {
        return key;
    }

    public void setKey(CustomFields key) {
        this.key = key;
    }

    @XmlElement(name="value")
    public String getValue() {
        return value;
    }

    public void setValue(String value) {
        this.value = value;
    }
}

MapAdapter.java

public class MapAdapter extends XmlAdapter<MapElements[], Map<CustomFields, String>> {

    @Override
    public Map<CustomFields, String> unmarshal(MapElements[] elements) throws Exception {

        HashMap<CustomFields, String> mapElements = new HashMap<>();

        for (MapElements element : elements) {
            if(!element.getValue().isEmpty())
                mapElements.put(element.getKey(), element.getValue());
        }

        return mapElements;
    }

    @Override
    public MapElements[] marshal(Map<CustomFields, String> elements) throws Exception {
        MapElements[] mapElements = new MapElements[elements.size()];
        int i = 0;
        for (Map.Entry<CustomFields, String> entry : elements.entrySet()) {
            mapElements[i++] = new MapElements(entry.getKey(), entry.getValue());
        }

        return mapElements;
    }
}

Test.java :

public class convertObjectToXml {


    public static void main(String[] args) throws JAXBException {

        Map<CustomFields, String> hashMap = new HashMap<>();
        hashMap.put(CustomFields.CODE, "213");
        hashMap.put(CustomFields.PHONE, "4356");

        Issue issue = new Issue();
        issue.setDescription("adam");
        issue.setSubject("Subject");

        issue.setCustomFields(hashMap);

        System.out.println(convertObjectToXml(issue));
    }
}

CustomFields is Enum :

public enum CustomFields {

    @XmlEnumValue("1")
    CODE(1),
    @XmlEnumValue("2")
    PHONE(2),
    ....
}

Nawet najmniejsza podpowiedź??