diff --git a/src/vnet/interface_output.c b/src/vnet/interface_output.c
index 73c7a31..eadee69 100644
--- a/src/vnet/interface_output.c
+++ b/src/vnet/interface_output.c
@@ -189,7 +189,10 @@
 	  th->checksum = ip4_tcp_udp_compute_checksum (vm, b, ip4);
 	}
       else if (b->flags & VNET_BUFFER_F_OFFLOAD_UDP_CKSUM)
-	uh->checksum = ip4_tcp_udp_compute_checksum (vm, b, ip4);
+	{
+	  uh->checksum = 0;
+	  uh->checksum = ip4_tcp_udp_compute_checksum (vm, b, ip4);
+	}
     }
   else if (is_ip6)
     {