Custom smart search by culture

I have a custom search index. Now search index is working but when I add a culture condition it returns null. Here is a part of index where I set culture field:

                        var documentParameters = new SearchDocumentParameters
                        {
                            Index = srchInfo,
                            Type = SearchHelper.CUSTOM_SEARCH_INDEX,
                            Id = Guid.NewGuid().ToString(),
                            Created = DateTime.Now,
                            SiteName = currentSiteName,
                            Culture = siteCulture.CultureCode
                        };

And part of combining search query:

        var searchCondition = culture != null
            ? SearchSyntaxHelper.GetFieldCondition("_culture", culture.ToLower(), true)
            : "";

        var condition = new SearchCondition(searchCondition, SearchModeEnum.AnyWord,
            SearchOptionsEnum.FullSearch);

As a result I get a query like: "+_culture:en-gb +_customfield:(term1 OR term2)"

This query will return null, but if I remove +_culture:en-gb then it will return all items(fro all cultures) according to customfield condition. I have to add that search documents have field "_culture" field.

How can I include filter by culture?