diff --git a/extmod/vfs_lfsx_file.c b/extmod/vfs_lfsx_file.c
index ab5cce50088b..56daa53e0686 100644
--- a/extmod/vfs_lfsx_file.c
+++ b/extmod/vfs_lfsx_file.c
@@ -90,9 +90,9 @@ mp_obj_t MP_VFS_LFSx(file_open)(mp_obj_t self_in, mp_obj_t path_in, mp_obj_t mod
     }
 
     #if LFS_BUILD_VERSION == 1
-    MP_OBJ_VFS_LFSx_FILE *o = mp_obj_malloc_var_with_finaliser(MP_OBJ_VFS_LFSx_FILE, uint8_t, self->lfs.cfg->prog_size, type);
+    MP_OBJ_VFS_LFSx_FILE *o = mp_obj_malloc_var_with_finaliser(MP_OBJ_VFS_LFSx_FILE, file_buffer, uint8_t, self->lfs.cfg->prog_size, type);
     #else
-    MP_OBJ_VFS_LFSx_FILE *o = mp_obj_malloc_var_with_finaliser(MP_OBJ_VFS_LFSx_FILE, uint8_t, self->lfs.cfg->cache_size, type);
+    MP_OBJ_VFS_LFSx_FILE *o = mp_obj_malloc_var_with_finaliser(MP_OBJ_VFS_LFSx_FILE, file_buffer, uint8_t, self->lfs.cfg->cache_size, type);
     #endif
     o->vfs = self;
     #if !MICROPY_GC_CONSERVATIVE_CLEAR
diff --git a/py/obj.h b/py/obj.h
index de740bf4ccf4..72def663d8d3 100644
--- a/py/obj.h
+++ b/py/obj.h
@@ -947,11 +947,11 @@ void *mp_obj_malloc_helper(size_t num_bytes, const mp_obj_type_t *type);
 // Object allocation macros for allocating objects that have a finaliser.
 #if MICROPY_ENABLE_FINALISER
 #define mp_obj_malloc_with_finaliser(struct_type, obj_type) ((struct_type *)mp_obj_malloc_with_finaliser_helper(sizeof(struct_type), obj_type))
-#define mp_obj_malloc_var_with_finaliser(struct_type, var_type, var_num, obj_type) ((struct_type *)mp_obj_malloc_with_finaliser_helper(sizeof(struct_type) + sizeof(var_type) * (var_num), obj_type))
+#define mp_obj_malloc_var_with_finaliser(struct_type, var_field, var_type, var_num, obj_type) ((struct_type *)mp_obj_malloc_with_finaliser_helper(offsetof(struct_type, var_field) + sizeof(var_type) * (var_num), obj_type))
 void *mp_obj_malloc_with_finaliser_helper(size_t num_bytes, const mp_obj_type_t *type);
 #else
 #define mp_obj_malloc_with_finaliser(struct_type, obj_type) mp_obj_malloc(struct_type, obj_type)
-#define mp_obj_malloc_var_with_finaliser(struct_type, var_type, var_num, obj_type) mp_obj_malloc_var(struct_type, var_type, var_num, obj_type)
+#define mp_obj_malloc_var_with_finaliser(struct_type, var_field, var_type, var_num, obj_type) mp_obj_malloc_var(struct_type, var_field, var_type, var_num, obj_type)
 #endif
 
 // These macros are derived from more primitive ones and are used to