MongoDB Indexes.CreateOneAsync Exception using Azure DocumentDB

I'm trying to create multiple unique indexes using c# MongoDB driver connecting to Azure DocumentDB instance, but I'm receiving the following exception when trying to create the second unique index:

MongoDB.Driver.MongoCommandException: 'Command createIndexes failed: Message: {"Errors":["The number of unique keys cannot be greater than 1."]}

I can't seem to find any documentation regarding the number of unique keys for Azure DocumentDB collection. Note that this exception does not occur when using actual MongoDB instance.

var keys = Builders<ProductEntity>.IndexKeys.Ascending(p => p.UPC);
var options = new CreateIndexOptions<ProductEntity>() { Name = "UX_UPC", Unique = true, Sparse = true };
var result = await _collection.Indexes.CreateOneAsync(keys, options);

keys = Builders<ProductEntity>.IndexKeys.Ascending(p => p.Manufacturer).Ascending(p => p.MPN);
options = new CreateIndexOptions<ProductEntity>() { Name = "UX_Manufacturer_MPN", Unique = true, Sparse = true };
result = await _collection.Indexes.CreateOneAsync(keys, options);

public class ProductEntity
{
    public Guid Id { get; set; }
    public string UPC { get; set; }
    public string MPN { get; set; }
    public string Manufacturer { get; set; }
}