A Field Or Property With The Name 'Jan' Was Not Found On The Selected Data Source.

I have this Stored Procedure:

Create PROCEDURE ListEvent
as
 If MONTH(GetDate()) <= 6
 Begin
SELECT EventTitle,  EventDuration,
  (CASE WHEN MONTH(StartDate) = 1 THEN Datename(Day,StartDate) + ' - ' + Datename(Day,EndDate) ELSE '' END) AS 'Jan',
  (CASE WHEN MONTH(StartDate) = 2 THEN Datename(Day,StartDate) + ' - ' + Datename(Day,EndDate) ELSE '' END) AS 'Feb',
  (CASE WHEN MONTH(StartDate) = 3 THEN Datename(Day,StartDate) + ' - ' + Datename(Day,EndDate) ELSE '' END) AS 'Mar',
  (CASE WHEN MONTH(StartDate) = 4 THEN Datename(Day,StartDate) + ' - ' + Datename(Day,EndDate) ELSE '' END) AS 'Apr',
  (CASE WHEN MONTH(StartDate) = 5 THEN Datename(Day,StartDate) + ' - ' + Datename(Day,EndDate) ELSE '' END) AS 'May',
  (CASE WHEN MONTH(StartDate) = 6 THEN Datename(Day,StartDate) + ' - ' + Datename(Day,EndDate) ELSE '' END) AS 'Jun'
FROM dbo.tblEvent INNER JOIN  dbo.tbl ON (tblEvent.EventID = tblEventdate.EventID)
WHERE  YEAR(StartDate) = Year(GetDate())
group by EventTitle, EventDuration,StartDate,EndDate
End 
Else 
Begin
SELECT EventTitle, EventDuration,
(CASE WHEN MONTH(StartDate) = 7 THEN Datename(Day,StartDate) + ' - ' + Datename(Day,EndDate) ELSE '' END) AS 'Jul',
  (CASE WHEN MONTH(StartDate) = 8 THEN Datename(Day,StartDate) + ' - ' + Datename(Day,EndDate) ELSE '' END) AS 'Aug',
  (CASE WHEN MONTH(StartDate) = 9 THEN Datename(Day,StartDate) + ' - ' + Datename(Day,EndDate) ELSE '' END) AS 'Sep',
  (CASE WHEN MONTH(StartDate) = 10 THEN Datename(Day,StartDate) + ' - ' + Datename(Day,EndDate) ELSE '' END) AS 'Oct',
  (CASE WHEN MONTH(StartDate) = 11 THEN Datename(Day,StartDate) + ' - ' + Datename(Day,EndDate) ELSE '' END) AS 'Nov',
  (CASE WHEN MONTH(StartDate) = 12 THEN Datename(Day,StartDate) + ' - ' + Datename(Day,EndDate) ELSE '' END) AS 'Dec'
FROM dbo.tblEvent INNER JOIN  dbo.tbl ON (tblEvent.EventID = tblEventdate.EventID)
WHERE  YEAR(StartDate) = Year(GetDate())
group by EventTitle, EventDuration,StartDate,EndDate
End 

 When I execute it in the SQLExpress, the result returned as expected. But when I bind to Gridview I got this error:

A field or property with the name 'Jan' was not found on the selected data source. 

How do I solve this?  

 I am using drag and drop SQLDatasource to call the Stored Proc and VB.net is the language.  Thanks