HELP - Stored Procedure ReturnValue is always -1

I have the following test stored procedure:

CREATE PROCEDURE dbo.sp_TestSSIS (@a int)
AS
BEGIN
    SET NOCOUNT ON
    DECLARE @myval int;
    SET @myval = (SELECT MAX(TestValue) % 2 FROM Test_SSIS_SP)
    RETURN @myval
END

When I run it in SQL Server Management Studio I get the correct results - 0 for even number and 1 for odd.

In SSIS I use the Execute SQL Task with a OLE DB connection, ResultSet none and the following SQLStatement:

exec ? = sp_TestSSIS ?

and I have two parameters:

User::SPResult   ReturnValue   LONG   0   -1

User::SPDummy Input            LONG    1   -1

Everything I read says SPResult should contain either 0 or 1, but I'm getting -1 everytime. I've tried every which way I could think, including using OUTPUT paramters, but to no avail. What am I missing?