diff --git a/udp.c b/udp.c
index e79ca93..8181900 100644
--- a/udp.c
+++ b/udp.c
@@ -279,9 +279,10 @@ static void udp_invert_portmap(struct udp_fwd_ports *fwd)
 		      "Forward and reverse delta arrays must have same size");
 	for (i = 0; i < ARRAY_SIZE(fwd->f.delta); i++) {
 		in_port_t delta = fwd->f.delta[i];
+		in_port_t rport = i + delta;
 
 		if (delta)
-			fwd->rdelta[i + delta] = NUM_PORTS - delta;
+			fwd->rdelta[rport] = NUM_PORTS - delta;
 	}
 }