IF EXISTS (SELECT) on SQL Server

I have a code like this:

IF EXISTS (SELECT * FROM table WHERE id = @id)
BEGIN
        UPDATE  table
        SET     stock   = stock + @stock
        WHERE   id      = @id
END
ELSE
BEGIN
        INSERT INTO [table](
                    [id]
                   ,[name]
                   ,[stock])
             VALUES(
                    @id
                   ,@name
                   ,@stock)
END

But, that code doesn't working. I was trace many times, there is no reason to be wrong. Can somebody help me?

Thanks in advance