diff --git a/src/core/netif.c b/src/core/netif.c
index b2fb80c5..d998288f 100644
--- a/src/core/netif.c
+++ b/src/core/netif.c
@@ -300,7 +300,6 @@ netif_add(struct netif *netif,
   /* remember netif specific state information data */
   netif->state = state;
   netif->num = netif_num++;
-  LWIP_ASSERT("Netif num overflow, too many netifs or adds/removes", netif->num < 255);
   netif->input = input;
 
   NETIF_SET_HWADDRHINT(netif, NULL);
@@ -317,6 +316,11 @@ netif_add(struct netif *netif,
     return NULL;
   }
 
+  /* check that netif_num has not overflowed or that init callback
+  provided a valid number (we don't support 255 since that can't be
+  converted to an index) */
+  LWIP_ASSERT("Netif num overflow/invalid num", netif->num < 255);
+
   /* add this netif to the list */
   netif->next = netif_list;
   netif_list = netif;