[flexcoders] Urgent :: Custom Serialization exception

I am using Custom Serialization to serialize my Java Object (TreeNode class)
"recursively" to ActionScript.
But this fails giving the following exception:

ArgumentError: Error #2004: One of the parameters is invalid.
   at flash.utils::ObjectInput/readObject()
   at com.flex.dao::TreeNode/readExternal()

(mx.messaging.messages::ErrorMessage)#0
 body = (Object)#1
 clientId = (null)
 correlationId = "5D934A88-B385-627D-2445-258088A878CA"
 destination = ""
 extendedData = (null)
 faultCode = "Server.Acknowledge.Failed"
 faultDetail = "Was expecting mx.messaging.messages.AcknowledgeMessage, but
received null"
 faultString = "Didn't receive an acknowledge message"
 headers = (Object)#2
 messageId = "983E8914-DDAC-1ECA-C02A-2580CCD4A40F"
 rootCause = (null)
 timestamp = 0
 timeToLive = 0


=====================================================
The code of "ActionScript" and "Java" TreeNode classes is as under:

======= ActionScript ============
package com.flex.dao
{
   import mx.collections.ArrayCollection;
   import flash.utils.IExternalizable;
   import flash.utils.IDataInput;
   import flash.utils.IDataOutput;
   import mx.controls.Alert;

   [RemoteClass(alias="com.java.dao.TreeNode")]
   public class TreeNode implements IExternalizable
   {
       public var name : String;
       public var id : int;
       public var type : int;
       public var children : ArrayCollection;
       public var data : Object;

       public function readExternal(input:IDataInput) : void
       {
           id = input.readInt();
           name = input.readUTF();
           type = input.readInt();

           var childCount : int = input.readInt();
           if(childCount != 0)
           {
               children = new ArrayCollection();
               for(var i:int = 0; i<childCount;i++)
               {
                   var obj : Object = input.readObject();
                   children.addItem(obj);
               }
           }
           var hasData : int = input.readInt();
           if(hasData != 0)
               data = input.readObject();
       }

       public function writeExternal(output:IDataOutput) : void
       {
           output.writeInt(id);
           output.writeUTF(name);
           output.writeInt(type);
           if(children != null)
           {
               output.writeInt(children.length);
               for(var i:int = 0; i<children.length;i++)
                   output.writeObject(TreeNode(children.getItemAt(i)));
           }
           else
               output.writeInt(0);
           if(data != null)
           {
               output.writeInt(1);
               output.writeObject(data);
           }
           else
               output.writeInt(0);
       }

   }
}


======== Java ==========
public class TreeNode implements Externalizable

{
   public List children; //list of TreeNode
   public Object data;


   public TreeNode()
   {
   }

   public TreeNode(int id, String name, int type) {
       super(id, name, type);
       children = new ArrayList();

   }


   public void addChild(TreeNode child) {
         children.add(child);
   }


   public void writeExternal(ObjectOutput objectOutput) throws IOException
   {
       objectOutput.writeInt(id);
       objectOutput.writeUTF(name);
       objectOutput.writeInt(type);

       if(children != null && children.size() > 0)
       {
           System.out.println(name + " :: "+children.size());
           objectOutput.writeInt(children.size());
           for(int i = 0; i< children.size();i++)
               objectOutput.writeObject(children.get(i));
       }
       else
           objectOutput.writeInt(0);
       if(data != null)
       {
           objectOutput.writeInt(1);
           objectOutput.writeObject(data);
       }
       else
           objectOutput.writeInt(0);
   }

   public void readExternal(ObjectInput objectInput) throws IOException,
ClassNotFoundException
   {
       id = objectInput.readInt();
       name = objectInput.readUTF();
       type = objectInput.readInt();

       int childCount = objectInput.readInt();
       if(childCount != 0)
       {
           children = new ArrayList();
           for(int i=0; i<childCount; i++)
               children.add((TreeNode)objectInput.readObject());

       }
       int hasData = objectInput.readInt();
       if(hasData != 0)
           data = objectInput.readObject();

   }
}