Serializing invetory items data and InvalidOperationException exception

Hi, I have this base class.

 using System;
 using System.Collections.Generic;
 
 [Serializable]
 public class TestBase
 {
     public int value;
     public TestBase()
     {
     }
 
     public TestBase(int val)
     {
         value = val;
     }
 }

And this inherited class

 using System;
 using System.Collections.Generic;
 
 [Serializable]
 class TestChild : TestBase
 {
     public int childValue;
     public TestChild(int parentVal, int childVal): base(parentVal)
     {
         childValue = childVal;
     }
 }

Then I have a SerializbleDictionary you might have already seen somewhere on the internet, which I am using like SerializableDictionary. Code for serialization looks like this.

     public void WriteXml(System.Xml.XmlWriter writer)
     {
         XmlSerializer keySerializer = new XmlSerializer(typeof(TKey));
         XmlSerializer valueSerializer = new XmlSerializer(typeof(TValue));
 
 
         foreach (TKey key in this.Keys)
         {
             writer.WriteStartElement("item");
 
             writer.WriteStartElement("key");
             keySerializer.Serialize(writer, key);
             writer.WriteEndElement();
 
             writer.WriteStartElement("value");
             TValue value = this[key];
             valueSerializer.Serialize(writer, value);
             writer.WriteEndElement();
 
             writer.WriteEndElement();
         }
     }

This is obviously just model of problem, in real I am got some items and it would be nice to take advantages of polymorphism. However, when I try to serialize it, I get

System.InvalidOperationException: The type of the argument object 'TestChild' is not primitive.

Can someone explain me please what is the problem ? I have been googling around, found other people fighting with this exception but their problems seemed quite different.

Thank you in advance.