diff --git a/include/alisp/allocator.h b/include/alisp/allocator.h
index abfff3b..e5fad80 100644
--- a/include/alisp/allocator.h
+++ b/include/alisp/allocator.h
@@ -15,10 +15,14 @@
 
 typedef struct
 {
+  u64 padding : 56;
+  tag_t tag   : 8;
   u64 references;
-  tag_t tag : 8;
 } alloc_metadata_t;
 
+static_assert(sizeof(alloc_metadata_t) == 16,
+              "16 byte metadata required for alignment purposes");
+
 typedef struct
 {
   alloc_metadata_t metadata;