"Syntax error or access violation" on Data Flow Task OLE DB Data Source

I am implementing expression parameter for a SQL Server connection string (like this: http://danajaatcse.wordpress.com/2010/05/20/using-an-xml-configuration-file-and-expressions-in-an-ssis-package/)  and it works fine except when it reaches data flow task - OLE DB Source task. In this task, I execute a stored procedure like this: 

exec SelectFromTableA ?,?,?

The error message is this:

0xC0202009 at Data Flow Task, OLE DB Source [2]: SSIS Error Code DTS_E_OLEDBERROR.  An OLE DB error has occurred. Error code: 0x80004005.
An OLE DB record is available.  Source: "Microsoft OLE DB Provider for SQL Server"  Hresult: 0x80004005  Description: "Syntax error or access violation".
Error: 0xC004706B at Data Flow Task, SSIS.Pipeline: "OLE DB Source" failed validation and returned validation status "VS_ISBROKEN"

When I change the SQL command above with reading from table directly it works fine. I should also add that before changing connection string of the SQL data source to use expression, the SSIS package was working fine and I know that the connection string is fine because other tasks in the package works fine!

Any idea why?