diff --git a/udp.c b/udp.c
index c031a05..a3961bf 100644
--- a/udp.c
+++ b/udp.c
@@ -258,15 +258,16 @@ void udp_portmap_clear(void)
  */
 static void udp_invert_portmap(struct udp_port_fwd *fwd)
 {
-	int i;
+	unsigned int i;
 
 	static_assert(ARRAY_SIZE(fwd->f.delta) == ARRAY_SIZE(fwd->rdelta),
 		      "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[(in_port_t)i + delta] = NUM_PORTS - delta;
+			fwd->rdelta[rport] = NUM_PORTS - delta;
 	}
 }