Deserialize empty XML element using XStream

In an XML stream i receive, I have the following statement

<user>
    <age/>
</user>

which is to be inserted in an object that looks like this :

@XStreamAlias("user")
public class User {

    public int age = 0;
}

Unfortunatly, I receive XStream exceptions each time I try to read this XML, as the age xml tag is empty :

Exception in thread "main" com.thoughtworks.xstream.converters.ConversionException: For input string: "" : For input string: ""
---- Debugging information ----
message             : For input string: ""
cause-exception     : java.lang.NumberFormatException
cause-message       : For input string: ""
class               : java.lang.Integer
required-type       : java.lang.Integer
converter-type      : com.thoughtworks.xstream.converters.SingleValueConverterWrapper
wrapped-converter   : com.thoughtworks.xstream.converters.basic.IntConverter
path                : /GoodreadsResponse/user/age
line number         : 17
class[1]            : fr.riduidel.exporter.goodreads.User
converter-type[1]   : com.thoughtworks.xstream.converters.reflection.ReflectionConverter
class[2]            : fr.riduidel.exporter.goodreads.GoodreadsResponse
version             : null
-------------------------------
    at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:79)
    at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:65)
    at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:66)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.unmarshallField(AbstractReflectionConverter.java:355)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.doUnmarshal(AbstractReflectionConverter.java:306)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.unmarshal(AbstractReflectionConverter.java:234)
    at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:72)
    at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:65)
    at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:66)
    at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:50)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.doUnmarshal(AbstractReflectionConverter.java:322)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.unmarshal(AbstractReflectionConverter.java:234)
    at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:72)
    at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:65)
    at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:66)
    at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:50)
    at com.thoughtworks.xstream.core.TreeUnmarshaller.start(TreeUnmarshaller.java:134)
    at com.thoughtworks.xstream.core.AbstractTreeMarshallingStrategy.unmarshal(AbstractTreeMarshallingStrategy.java:32)
    at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:1052)
    at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:1036)
    at com.thoughtworks.xstream.XStream.fromXML(XStream.java:921)

How can I tell XStream to consider this field as "optional" or "possibliy containing nothing" ?