diff --git a/src/core/ipv6/ip6_frag.c b/src/core/ipv6/ip6_frag.c
index d6c5d223..8b352f56 100644
--- a/src/core/ipv6/ip6_frag.c
+++ b/src/core/ipv6/ip6_frag.c
@@ -781,7 +781,7 @@ ip6_frag(struct pbuf *p, struct netif *netif, const ip6_addr_t *dest)
       return ERR_MEM;
     }
     LWIP_ASSERT("this needs a pbuf in one piece!",
-                (p->len >= (IP6_HLEN)));
+                (rambuf->len >= (IP6_HLEN)));
     SMEMCPY(rambuf->payload, original_ip6hdr, IP6_HLEN);
     ip6hdr = (struct ip6_hdr *)rambuf->payload;
     frag_hdr = (struct ip6_frag_hdr *)((u8_t*)rambuf->payload + IP6_HLEN);