Slurs in multiple voices

[Cris Kenton]

Hello,

I have the following code:
{
                r4 <fas si res>4 <si res fas>4) |
                r4 <fas si res>4 <res' fas si> |
        } \\%% End upper voice
        {
                si,,2.( | res2. |
        }%% End lower voice


I would like to connect the slur starting in the lower voice with the last
chord in the upper voice. I got an error. Please, help! :)
thanks,

Cris