Tooltip error msg

The tooltip isn't showing in my chart (I selected it in Series Properties) and I am getting an error msg:

Warning	1	[rsInvalidExpressionDataType] The ToolTip expression used in chart ‘Chart3’ returned a data type that is not valid.	C:\Users\dr\Documents\Visual Studio 2008\Projects\Report Project2\Report Project2\Report1.rdl	0	0	

I am only using Expr1 in the chart data values so I don't think it's anything to do with CAST or ROUND in Expr2.

SELECT        t_Employees.EmployeeSurname, SUM(t_PointsSystem.Points) AS Expr1, CAST(ROUND(SUM(t_PointsSystem.Points) * 1.2, 0) AS INT) AS Expr2
FROM            t_Employees INNER JOIN
                         ln_DriversPoints ON t_Employees.EmployeeID = ln_DriversPoints.EmployeeID INNER JOIN
                         t_PointsSystem ON ln_DriversPoints.PointsID = t_PointsSystem.PointsID
GROUP BY t_Employees.EmployeeSurname
ORDER BY Expr1 DESC

Any ideas?