使用ASP.NET WEB Services返回列的总和

I have been tring to return the sum of a column using this web service but it does not work .Please Help.Thank You. I am using Visual Studio 2010.It Just Keeps on Returning an error.System.InvalidOperationException: There was an error generating the XML document. ---> System.InvalidOperationException: Cannot serialize the DataTable. I have been tring to return the sum of a column