diff --git a/p4src/checksum_diff.p4 b/p4src/checksum_diff.p4
index beb96da..b5f7def 100644
--- a/p4src/checksum_diff.p4
+++ b/p4src/checksum_diff.p4
@@ -623,7 +623,7 @@ Echo or Echo Reply Message
     action v4sum() {
         bit<16> localsum = 0;
 
-        localsum += hdr.ipv4.src[15:0];              // 16 bit
+        localsum = localsum + hdr.ipv4.src[15:0];              // 16 bit
         localsum += hdr.ipv4.src[31:16];             // 16 bit
         localsum += hdr.ipv4.dst[15:0];              // 16 bit
         localsum += hdr.ipv4.dst[31:16];             // 16 bit