SSRS 2008 not returning correct dataset from Stored Procedure

Hi,

I have a stored proc which returns different no. of columns as output based on the input parameter. The issue I am facing is that the SSRS 2008 is always populating the column list of the dataset with the first select list irrespective of the input parameter.

How can I have the SSRS populate the column list based on the input parameter I am passing?

eg.

IF @type='test'

BEGIN

SELECT 1 AS Col1,2 AS Col2

END

ELSEIF @type='test1'

BEGIN

SELECT 1 AS Col1,2 AS Col2,3 AS Col3

SSRS always populates the dataset with two columns  viz col1 & col2 even when I pass'test1' as input parameter.

Thanks for the help in advance.

Sherin