C# MongoDB driver: unique index

I have a model with _id, name and other fields. I want to create an index so it would be impossible to save documents with same names. I'm creating an index such way:

public static async Task EnsureIndexExists(this IMongoDatabase database, string collectionName, string indexName)
            var collection = database.GetCollection<BsonDocument>(collectionName);
            var builder = Builders<BsonDocument>.IndexKeys;
            var field = new StringFieldDefinition<BsonDocument>(indexName.FromPascalToCamel());
            var indexModel = new CreateIndexModel<BsonDocument>(builder.Ascending(field));
            await collection.Indexes.CreateOneAsync(indexModel).ConfigureAwait(false);

// ...

database.EnsureIndexExists(CharacterRepository.CollectionName, nameof(CharacterEntity.Name)).Wait();

The model:

public class CharacterEntity
        public string Id { get; set; }
        public string Name { get; set; }

But still documents with the same names can be saved into DB.

The driver versions:

<PackageReference Include="AspNetCore.Identity.MongoDbCore" Version="2.1.1" />
<PackageReference Include="MongoDB.Driver" Version="2.11.1" />

What I'm doing wrong?