c# binary serialization "unexpected binary element: 225"

I am trying to save my custom class using binary serialization in c#. I have the serialization working fine (as far as I can tell, I get no errors) but when I try to deserialize it I get the following error:

SerializationException: Unexpected binary element: 255

Does anybody got ideas as to what could be causing this? I'm using C# with the Unity game engine.

EDIT:

Here's the code, it's inside the class for my editor window.

public static LevelMetaData LoadAllLevels()
{
    string filepath = Path.Combine(Application.dataPath, "Levels/meta.dat");

    BinaryFormatter serializer = new BinaryFormatter();

    if (File.Exists(filepath))
    {
        using (StreamReader sr = new StreamReader(filepath))
        {
            return (LevelMetaData)serializer.Deserialize(sr.BaseStream);
        }
    }
    else
    {
        LevelMetaData temp = new LevelMetaData();

        using (StreamWriter sw = new StreamWriter(filepath))
        {
            serializer.Serialize(sw.BaseStream, temp);
        }
        return temp;
    }
}

EDIT 2:

Here's the LevelMetaData class:

[Serializable]
public class LevelMetaData
{
    public LevelMetaData()
    {
        keys = new List<string>();
        data = new List<LevelController>();
    }

    public LevelController this[string key]
    {
        get
        {
            if (keys.Contains(key))
                return data[keys.IndexOf(key)];
            else
                throw new KeyNotFoundException("Level with key \"" + key + "\"not found.");
        }
    }

    public void Add(string key, LevelController level)
    {
        if (!keys.Contains(key))
        {
            keys.Add(key);
            data.Add(level);
        }
    }

    public bool Contains(string key)
    {
        return keys.Contains(key);
    }

    public List<string> keys;
    public List<LevelController> data;
}