Best bets not working

Hi,

I need to apply best bets to my site.In best bets Target content I choose commerce catalog product.While I tried to get result from beat bets by using following code getting error says "Culture is not supported".

  var results = SearchClient.Instance.UnifiedSearchFor(spaceText).ApplyBestBets().GetResult();
                var result = SearchClient.Instance.Search()
                                .For(spaceText)
                                .ApplyBestBets().GetResult();
////For both cases I am getting the same error.

Thanks in advance.