diff --git a/lib/birdlib.h b/lib/birdlib.h
index 431b7c0d..2e642d38 100644
--- a/lib/birdlib.h
+++ b/lib/birdlib.h
@@ -16,7 +16,7 @@
 struct align_probe { char x; long int y; };
 
 #define OFFSETOF(s, i) ((size_t) &((s *)0)->i)
-#define SKIP_BACK(s, i, p) ((s *)((char *)p - OFFSETOF(s, i)))
+#define SKIP_BACK(s, i, p) ({ s *_ptr = ((s *)((char *)p - OFFSETOF(s, i))); ASSERT_DIE(&_ptr->i == p); _ptr; })
 #define BIRD_ALIGN(s, a) (((s)+a-1)&~(a-1))
 #define CPU_STRUCT_ALIGN (sizeof(struct align_probe))