How to sort search result on the basis of other fields

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C#

ASP.NET

Lucene.NET

Hi,
I am using lucene.net for fulltext searching, Now I want to sort searched result through my date column as desc. I am usig following piece of code but it is returing null:

Sort sort = new Sort(new SortField[] { new SortField("publishdate", true) });
hits = searcher.Search(query,sort );

The above variable hits is getting null.