Serializable fields of serializable class lost after application reload

Hello,

We are having some serialization problems. We have three types, see attached script below. There is a SerializableDictionary that has itself two references to other classes derived off of SerializableDictionary.

We create an instance of the base dictionary (AdditionalParametersCache) and store it in a ScriptableObject like so:

Code (CSharp):

1. [HideInInspector]
2. [SerializeField]
3. public AdditionalParametersCache m_AdditionalParametersCache;
4.  

Which is then initialised like this:

Code (CSharp):

1. m_AdditionalParametersCache = new AdditionalParametersCache();
2. m_AdditionalParametersCache[typeof(int)] = new IntegerParametersCache();
3. m_AdditionalParametersCache[typeof(string)] = new StringParametersCache();

This works fine until we close the editor and then reopen the project. The base dictionary still persists, but its two entries are now null. I tried to read up on how unity handles serialization but can't figure out how this happens. All other serialisable fields of the scriptable object are persisted just fine.

Code (CSharp):

1. using System;
2. using System.Collections.Generic;
3. using System.IO;
4. using System.Security.Cryptography;
5. using XXX.BuildTools.Misc;
6. using XXX.BuildTools.SerializableDictionary;
7.  
8. namespace XXX.BuildTools.Classes
9. {
10.     [Serializable]
11.     public class AdditionalParametersCache : SerializableDictionary<Type, SerializableDictionaryBase> {};
12.  
13.     [Serializable]
14.     public class IntegerParametersCache : SerializableDictionary<string, int> {};
15.  
16.     [Serializable]
17.     public class StringParametersCache : SerializableDictionary<string, string> {};
18. }
19.