FormulaFields only accepts numeric data

Hi,

I have a problem. I am passing text in FormulaFields. It passes only numeric data.

ReportDocument rptdoc = new ReportDocument();
rptdoc.Load(Server.MapPath("CrystalReport2.rpt"));
rptdoc.SetDataSource(ds);
this.CrystalReportViewer1.RefreshReport();
this.CrystalReportViewer1.ReportSource = rptdoc;
CrystalReportViewer1.ToolPanelView = CrystalDecisions.Web.ToolPanelViewType.None;
rptdoc.DataDefinition.FormulaFields[1].Text = "12";     //this is passing to the report and showing in report "12.00" and is right aligned
rptdoc.DataDefinition.FormulaFields[1].Text = "abc";    //this is not passing and showing bellow error
aConnection.Close();

The remaining text does not appear to be part of the formula. Details: errorKind

Why this happening? Name of formula on index 1 in crystal report is @AdSpaces. I tried changing 

rptdoc.DataDefinition.FormulaFields[1].Text = "abc";

into 

rptdoc.DataDefinition.FormulaFields["AdSpaces"].Text = "abc";

but same error. Please help me I am wasted much time with this. Thanks