Developer forum

Seems like the indexer cannot find the product of a given productid. Can be a variant where a master does not exist, or where a given language of a product does not exist in default language.

Can also be that you have an integration running at the same time that removes some of the products as they are being indexed.

Exceptions is from here:

private void HandlePrices(string productId, string variantId, string languageId, IndexDocument document)
        {
            Product product = null;
            if (!_priceProducts.TryGetValue(productId, out product))
            {
                product = Services.Products.GetProductById(productId, "", true);
                _priceProducts[productId] = product;
            }

            if (product is null)
                throw new Exception("Invalid product data. Master product does not exist in default language.");