diff --git a/src/core/ipv4/ip4_frag.c b/src/core/ipv4/ip4_frag.c
index 9812a085..c1201cda 100644
--- a/src/core/ipv4/ip4_frag.c
+++ b/src/core/ipv4/ip4_frag.c
@@ -785,7 +785,7 @@ ip4_frag(struct pbuf *p, struct netif *netif, const ip4_addr_t *dest)
         p = p->next;
       }
     }
-    poff = newpbuflen;
+    poff += newpbuflen;
 #endif /* LWIP_NETIF_TX_SINGLE_PBUF */
 
     /* Correct header */