diff --git a/lib/birdlib.h b/lib/birdlib.h
index b7cd6b43..12a581b7 100644
--- a/lib/birdlib.h
+++ b/lib/birdlib.h
@@ -13,7 +13,7 @@
 
 /* Ugly structure offset handling macros */
 
-#define OFFSETOF(s, i) ((unsigned int)&((s *)0)->i)
+#define OFFSETOF(s, i) ((size_t) &((s *)0)->i)
 #define SKIP_BACK(s, i, p) ((s *)((char *)p - OFFSETOF(s, i)))
 #define BIRD_ALIGN(s, a) (((s)+a-1)&~(a-1))