diff --git a/sys/net/crosslayer/ng_pktbuf_static/ng_pktbuf_static.c b/sys/net/crosslayer/ng_pktbuf_static/ng_pktbuf_static.c
index 69cc65d5b5..58336ab120 100644
--- a/sys/net/crosslayer/ng_pktbuf_static/ng_pktbuf_static.c
+++ b/sys/net/crosslayer/ng_pktbuf_static/ng_pktbuf_static.c
@@ -429,6 +429,8 @@ static inline bool _too_small_hole(_unused_t *a, _unused_t *b)
 
 static inline _unused_t *_merge(_unused_t *a, _unused_t *b)
 {
+    assert(b != NULL);
+
     a->next = b->next;
     a->size = b->size + ((uint8_t *)b - (uint8_t *)a);
     return a;