SSIS 2012 and sp_describe_first_result_set

Hi,

I have a stored procedure with conditional logic that returns data for an SSIS datasource.  The various conditions in the sproc return the same column names and datatypes, but they still fail the "sp_describe_first_result_set" call from SSIS. After some digging around, I discovered that one of the conditions returns a "hidden" column that is used for lookup purposes, and this is not contained in the other conditions.  How can I resolve this so that SSIS doesn't complain about metadata not matching up?

The conditional query in the sproc is below:

	if @RunTypeLoc <> 'REL' and @RowVersionMinLoc = 0
	begin
		SELECT	pk_col,
                xxx,
                yyy,
                zzz,
				CAST('' AS NCHAR(1)) AS sys_change_operation
		FROM [dbo].aaa;
	end

	else IF @RunTypeLoc = 'INC'
	begin
		SELECT	ct.pk_col,
                xxx,
                yyy,
                zzz,
				ct.sys_change_operation
		FROM [dbo].xxx x
			RIGHT OUTER JOIN CHANGETABLE(CHANGES dbo.xxx, @RowVersionMinLoc) AS ct ON x.pk_col= ct.pk_col
		WHERE   ct.sys_change_version <= @RowVersionMaxLoc;
	END

	else if @RunTypeLoc = 'REL'
	begin
		SELECT	pk_col,
                xxx,
                yyy,
                zzz,
				CAST('' AS NCHAR(1)) AS sys_change_operation
		FROM [dbo].xxx
		where yyy in (select yyy from aaa);
	end