Cannot draw commutative diagram

Question

Possible Duplicate:
Cannot draw diagram

My code is:

\xymatrix{& C \ar@/_/[ld]_f \ar[d]_{(f,g)} \ar@/^/[rd]^g\\
          & A\times B \ar[ld]_p \ar[rd]^q\\
          A && B}

and the error I got:

! Xy-pic error: in entry "1,2": No [rd] (is "2,3") from here.