--- a/mailbox/assoc.c
+++ b/mailbox/assoc.c
@@ -58,7 +58,7 @@
   mu_iterator_t itr;
 };
 
-#define ASSOC_ELEM_SIZE(a) ((a)->elsize + sizeof(struct _mu_assoc_elem) - 1)
+#define ASSOC_ELEM_SIZE(a) ((a)->elsize + sizeof(struct _mu_assoc_elem))
 
 #define __ASSOC_ELEM(a,p,n) \
  ((struct _mu_assoc_elem*) ((char*) (p) + ASSOC_ELEM_SIZE (a) * n))