Custom Serialization Issue

My remote method returns a java class and I'm trying to map that my flex class.  The remote method isn't throwing an exception.  That works fine, but on the flex end I get error message of: "faultCode:Server.Acknowledge.Failed faultString:'Didn't receive an acknowledge message' faultDetail:'Was expecting mx.messaging.messages.AcknowledgeMessage, but received null'".

I made sure the classes on both ends are Externalizable. i mapped the flex class to the java class.  Both sides read and write the correct data type. My swf does reference the class, so flex should be able to find it. i can't figure out why I'm getting the error message.

///////////Flex Class//////
package MyData
{
      import flash.utils.IDataInput;
      import flash.utils.IDataOutput;
      import flash.utils.IExternalizable;

      //[Bindable]
      [RemoteClass(alias="com.netcsd.flex.datamodel.ChatRoomData")]
      public class ChatRoom implements IExternalizable
      {
            public var chatRoomId:String;
            public var chatRoomName:String;
            public function ChatRoom()
            {
                  chatRoomId = "";
                  chatRoomName = "";
            }
            
            public function readExternal(input:IDataInput):void{
                  chatRoomId = input.readUTF();
                  chatRoomName = input.readUTF();
            }
            
            public function writeExternal(output:IDataOutput):void{
                  output.writeUTF(chatRoomId);
                  output.writeUTF(chatRoomName);
            }
      }
}

////////////////Java Class///////////
package com.netcsd.flex.datamodel;

import java.io.Externalizable;
import java.io.IOException;
import java.io.ObjectInput;
import java.io.ObjectOutput;

public class ChatRoomData implements Externalizable {
      private String chatRoomId;
      private String chatRoomName;
      
      public ChatRoomData(){
            chatRoomId ="";
            chatRoomName = "";
      }
      
      public String getChatRoomId(){
            return chatRoomId;
      }
      
      public String getChatRoomName(){
            return chatRoomName;
      }
      
      public void setChatRoomId(String id){      
            chatRoomId = id;
      }
      
      public void setChatRoomName(String name){
            chatRoomName = name;
      }
      
      public void readExternal(ObjectInput in) throws IOException,
    ClassNotFoundException {
            // Read in the server properties from the client representation.
            chatRoomId = in.readUTF();
            chatRoomName = in.readUTF();
    }
      
      public void writeExternal(ObjectOutput out) throws IOException{
            out.writeUTF(chatRoomId);
            out.writeUTF(chatRoomName);
      }
}