Struts2とTypeConvertionで、以下の場合に変換されないでデフォルトが使われてしまう

SampleValueObject-conversion.properties

Element_birthday=lumi2.type.CalendarConverter

SampleValueObject.java

@Validations(requiredStrings = {
    @RequiredStringValidator(fieldName = "username", message = "ユーザー名は必須入力です"),
}, intRangeFields = {
    @IntRangeFieldValidator(fieldName = "age", min = "20", max = "99", message = "年齢は20～99の値で入力してください")
}, regexFields = {
    @RegexFieldValidator(fieldName = "telephone", regex = "\\d{2,5}-\\d{1,4}-\\d{4,5}", message = "電話番号のフォーマットが正しくありません"),
})
public @Data
class SampleValueObject implements Serializable {

    private static final long serialVersionUID = 8751205553661194912L;

    private String username;
    private int age;
    private String telephone;
    @Element(value = java.util.Calendar.class)
    private List<Calendar> birthday;
}

struts2.log

Setting params struts.validateOnly => [ true ] param.telephone => [  ] param.age => [ 22 ] param.username => [  ] param.birthday[1] => [ 2012/PP/13 ] param.birthday[0] => [ 2015/5/5 ] struts.enableJSONValidation => [ true ] 
.......
2013-04-23 18:33:02,274 DEBUG [com.opensymphony.xwork2.conversion.impl.InstantiatingNullHandler] - Entering nullPropertyValue [target=SampleValueObject(username=null, age=22, telephone=null, birthday=null), property=birthday]
2013-04-23 18:33:02,274 DEBUG [com.opensymphony.xwork2.conversion.impl.XWorkConverter] - Retrieving convert for class [class lumi2.vo.SampleValueObject] and property [param.birthday.null]
2013-04-23 18:33:02,274 DEBUG [com.opensymphony.xwork2.conversion.impl.XWorkConverter] - Converter is null for property [param.birthday.null]. Mapping size [1]: