diff --git a/src/core/ipv6/ip6_frag.c b/src/core/ipv6/ip6_frag.c
index bfff50c2..e670e3da 100644
--- a/src/core/ipv6/ip6_frag.c
+++ b/src/core/ipv6/ip6_frag.c
@@ -112,6 +112,11 @@ ip6_reass_tmr(void)
 {
   struct ip6_reassdata *r, *tmp;
 
+#if !IPV6_FRAG_COPYHEADER
+  LWIP_ASSERT("sizeof(struct ip6_reass_helper) <= IP6_FRAG_HLEN, set IP6_FRAG_COPYHEADER to 1",
+    sizeof(struct ip6_reass_helper) <= IP6_FRAG_HLEN);
+#endif /* !IPV6_FRAG_COPYHEADER */
+
   r = reassdatagrams;
   while (r != NULL) {
     /* Decrement the timer. Once it reaches 0,