diff --git a/src/vnet/ip/ip4_format.c b/src/vnet/ip/ip4_format.c
index 6c31b42..eebd5ad 100644
--- a/src/vnet/ip/ip4_format.c
+++ b/src/vnet/ip/ip4_format.c
@@ -165,7 +165,7 @@
 
     /* Fragment offset. */
     o = 8 * (f & 0x1fff);
-    f ^= o;
+    f ^= f & 0x1fff;
     if (o != 0)
       s = format (s, " offset %d", o);