Unhandled exception. MongoDB.Driver.Linq.ExpressionNotSupportedException: Expression must be a constant: value(Program+<>c__DisplayClass0_0).key in o.Keys.get_Item(value(Program+<>c__DisplayClass0_0).key).

using MongoDB.Bson;
using MongoDB.Driver;
using MongoDB.Driver.Linq;
// Init.
var mongoUrl = new MongoUrl("mongodb://localhost");
var settings = MongoClientSettings.FromUrl(mongoUrl);
settings.LinqProvider = LinqProvider.V3; // WORKS WITH V2.
var client = new MongoClient(settings);
var database = client.GetDatabase("default");
var collection = database.GetCollection<Model>("Models");

// Insert document with key.
var id = ObjectId.GenerateNewId();
var key = "key";
var model = new Model(id, new Dictionary<string, string> { [key] = "value" });
collection.InsertOne(model);

// ✅ Update value.
collection.UpdateOne(
    Builders<Model>.Filter.Eq(o => o.Id, id),
    new UpdateDefinitionBuilder<Model>().Set(o => o.Keys[key], "new value"));

// ❌ Remove key.
collection.UpdateOne(
    Builders<Model>.Filter.Eq(o => o.Id, id),
    new UpdateDefinitionBuilder<Model>().Unset(o => o.Keys[key]));


record Model(ObjectId Id, Dictionary<string, string> Keys);