diff --git a/ipv4.c b/ipv4.c
index b5cbf80..1d5b0b2 100644
--- a/ipv4.c
+++ b/ipv4.c
@@ -70,7 +70,7 @@
   uint8_t nxthdr;
   const char *next_header;
   size_t len_left;
-  uint32_t checksum;
+  uint32_t old_sum, new_sum;
   int iov_len;
 
   if(len < sizeof(struct iphdr)) {
@@ -121,14 +121,17 @@
   out[pos].iov_len = sizeof(struct ip6_hdr);
 
   // Calculate the pseudo-header checksum.
-  checksum = ipv6_pseudo_header_checksum(0, ip6_targ, len_left);
+  old_sum = ipv4_pseudo_header_checksum(0, header, len_left);
+  new_sum = ipv6_pseudo_header_checksum(0, ip6_targ, len_left);
 
   if (nxthdr == IPPROTO_ICMPV6) {
-    iov_len = icmp_packet(out, pos + 1, (const struct icmphdr *) next_header, checksum, len_left);
+    iov_len = icmp_packet(out, pos + 1, (const struct icmphdr *) next_header, new_sum, len_left);
   } else if (nxthdr == IPPROTO_TCP) {
-    iov_len = tcp_packet(out, pos + 1, (const struct tcphdr *) next_header, checksum, len_left);
+    iov_len = tcp_packet(out, pos + 1, (const struct tcphdr *) next_header, old_sum, new_sum,
+                         len_left);
   } else if (nxthdr == IPPROTO_UDP) {
-    iov_len = udp_packet(out, pos + 1, (const struct udphdr *) next_header, checksum, len_left);
+    iov_len = udp_packet(out, pos + 1, (const struct udphdr *) next_header, old_sum, new_sum,
+                         len_left);
   } else if (nxthdr == IPPROTO_GRE) {
     iov_len = generic_packet(out, pos + 1, next_header, len_left);
   } else {