diff --git a/src/arena.c b/src/arena.c
index aa4bfa42..25ce56ec 100644
--- a/src/arena.c
+++ b/src/arena.c
@@ -51,12 +51,13 @@ typedef struct mi_arena_s {
   bool     exclusive;                     // only allow allocations if specifically for this arena
   bool     is_large;                      // memory area consists of large- or huge OS pages (always committed)
   _Atomic(size_t) search_idx;             // optimization to start the search for free blocks
-  _Atomic(mi_msecs_t) purge_expire;       // expiration time when blocks should be decommitted from `blocks_decommit`.
+  _Atomic(mi_msecs_t) purge_expire;       // expiration time when blocks should be decommitted from `blocks_decommit`.  
   mi_bitmap_field_t* blocks_dirty;        // are the blocks potentially non-zero?
   mi_bitmap_field_t* blocks_committed;    // are the blocks committed? (can be NULL for memory that cannot be decommitted)
   mi_bitmap_field_t* blocks_purge;        // blocks that can be (reset) decommitted. (can be NULL for memory that cannot be (reset) decommitted)
   mi_bitmap_field_t* blocks_abandoned;    // blocks that start with an abandoned segment. (This crosses API's but it is convenient to have here)
   mi_bitmap_field_t  blocks_inuse[1];     // in-place bitmap of in-use blocks (of size `field_count`)
+  // do not add further fields here as the dirty, committed, purged, and abandoned bitmaps follow the inuse bitmap fields.
 } mi_arena_t;