diff --git a/p4src/checksum_diff.p4 b/p4src/checksum_diff.p4
index ab6b503..cb9655f 100644
--- a/p4src/checksum_diff.p4
+++ b/p4src/checksum_diff.p4
@@ -209,7 +209,7 @@ control MyIngress(inout headers hdr,
                     v4sum();
                     v6sum();
                     bit<16> diff = meta.v6sum - meta.v4sum;
-                    hdr.tcp.checksum = hdr.tcp.checksum + ~diff;
+                    hdr.tcp.checksum = hdr.tcp.checksum + ~diff +1;
 
                     #else