Specified cast is not valid

Hello, I have the following code:

if (dataTable.Columns.Count > 1)
                    {
                        strXML = ChartFileNameandPath(chartType) + @"?dataXML=<graph caption='Report ; Summary' subcaption='" + ReportName(reportType) + "' xAxisName='Line Number' yAxisName='Pounds made' rotatelabels='1' slantlabels='1' showNames='1' showvalues='0' numberPrefix='$' exportenabled='1' exportAtClient='1' exportHandler='fcExporter1' exportFormats='PNG=Save as PNG' >";
                        //Getting the Subtotal of PoundsMade based on the Line Number column
                        //C# linq query
                        if (reportType == ReportEnums.Clientsdata) {
                            var query = from row in dataTable.AsEnumerable()
                                        group row by row.Field<string>("ClientName") into grp
                                        orderby grp.Key
                                        select new {
                                            Clientname = grp.Key,
                                            TotalAmountSpent = grp.Sum(r => r.Field<int>("TotalAmountSpent")) //error here, specified cast is not valid
                                        };

                            foreach (var grp in query) {
                                strXML = strXML + "<set name='" + grp.Clientname.ToString() + "' value='" + grp.TotalAmountSpent.ToString() + "'/>";
                            }
                        } 

I get a error "Specified cast is not valid"

Part of my store procedure where I'm grabbing it from is:

SELECT  ClientName,
		Mem_Name,
		Mem_Address,
		Mem_city,
		Mem_state,
		Sum(mv.AmountSpent) as TotalAmountSpent,
		(Sum(mv.AmountSpent)/ (CAST(s.TotalBusinessSales as decimal)) * 100)  as PercentAmountSpent

My question is: Am I getting an error because of TotalAmountSpent is an alias?

Thanks,

N