diff --git a/p4src/actions_delta_checksum.p4 b/p4src/actions_delta_checksum.p4
index ad7eb44..c350829 100644
--- a/p4src/actions_delta_checksum.p4
+++ b/p4src/actions_delta_checksum.p4
@@ -68,13 +68,13 @@ action delta_prepare()
 action delta_udp_from_v4_to_v6()
 {
     delta_prepare();
-    hdr.udp.checksum = hdr.udp.checksum + ~meta.headerdiff;
+    hdr.udp.checksum = hdr.udp.checksum + ~meta.headerdiff -1;
 }
 
 action delta_tcp_from_v4_to_v6()
 {
     delta_prepare();
-    hdr.tcp.checksum = hdr.tcp.checksum + ~meta.headerdiff;
+    hdr.tcp.checksum = hdr.tcp.checksum + ~meta.headerdiff -1;
 }
 
 action delta_ipv4_from_v6_to_v4()