[Simple-support] Invalid reference '16' found. On empty String object (not null)

[Lorenzo D. C. <lor...@gm...>]

Hello, I'm trying to use Simple XML Serialization for saving a big 
object into files.
I have circular references, so I use CycleStrategy.

The problem is that if I save some fields that are empty Strings, it 
uses only one object, all the others are references:

<tables id="7">
  <entry>
     <string id="8">m_units</string>
     <tableInfo id="9">
        <fields id="10">
[...]
           <entry>
              <string id="14">descrizione</string>
              <fieldInfo id="15">
                 <fieldName reference="14"/>
                 <columnDefault id="16"></columnDefault>
                 <isNullable>false</isNullable>
                 <dataType id="17">STRING</dataType>
                 <parentTable reference="9"/>
              </fieldInfo>
           </entry>
[...]
           <entry>
              <string id="151">ragsoc</string>
              <fieldInfo id="152">
                 <fieldName reference="151"/>
                 <columnDefault reference="16"/>
                 <isNullable>false</isNullable>
                 <dataType reference="17"/>
                 <parentTable reference="127"/>
              </fieldInfo>
           </entry>
[...]

But when I try to load this I get: Invalid reference '16' found.
Am I doing something wrong?

the Object I'm trying to save is like this:

@Root
public class FieldInfo {
        @Element(required=false)
        public String fieldName;
        @Element(required=false)
        public String columnDefault;
        @Element(required=false)
        public boolean isNullable;
        @Element(required=false)
        public DATA_TYPE dataType;
        @Element(required=false)
        public TableInfo parentTable;
}