The example sritp on this page: https://docs.unity3d.com/2017.1/Documentation/ScriptReference/ISerializationCallbackReceiver.html
https://docs.unity3d.com/ScriptReference/ISerializationCallbackReceiver.html

should be changed
from:
public List<int> _keys = new List<int> { 3, 4, 5 };
public List<string> _values = new List<string> { "I", "Love", "Unity" };

//Unity doesn't know how to serialize a Dictionary
public Dictionary<int, string> _myDictionary = new Dictionary<int, string>();

to:
public List<int> _keys = new List<int>();
public List<string> _values = new List<string>();

//Unity doesn't know how to serialize a Dictionary
public Dictionary<int, string> _myDictionary = new Dictionary<int, string>{ { 3, "I" }, { 4, "Love" }, { 5, "Unity" } };