Summary

Please provide a clear and concise description of the bug.
Attempting to query on a field of type char? causes the driver to crash.

Please provide the version of the driver. If applicable, please provide the MongoDB server version and topology (standalone, replica set, or sharded cluster).

This is a regression in version 2.19. The same code runs fine without any problem in version 2.18.

How to Reproduce

Steps to reproduce. If possible, please include a Short, Self Contained, Correct (Compilable), Example.

using MongoDB.Driver;

MongoClient client = new MongoClient("mongodb://localhost:27017");
IMongoDatabase db = client.GetDatabase("test");
IMongoCollection<TestObject> coll = db.GetCollection<TestObject>("foo");
coll.Find(x => x.Prop == 'a').ToList();

public class TestObject
{
    public char? Prop { get; set; }
}

Additional Background

Please provide any additional background information that may be helpful in diagnosing the bug.