diff --git a/src/pipewire/map.h b/src/pipewire/map.h
index 1867fa4d3..f2b54fc7c 100644
--- a/src/pipewire/map.h
+++ b/src/pipewire/map.h
@@ -115,7 +115,7 @@ struct pw_map {
  */
 static inline void pw_map_init(struct pw_map *map, size_t size, size_t extend)
 {
-	pw_array_init(&map->items, extend);
+	pw_array_init(&map->items, extend * sizeof(union pw_map_item));
 	pw_array_ensure_size(&map->items, size * sizeof(union pw_map_item));
 	map->free_list = SPA_ID_INVALID;
 }