diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 573d0b20b76..f305dd31876 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -929,8 +929,10 @@ static struct object_entry **compute_write_order(void)
 	 */
 	for_each_tag_ref(mark_tagged, NULL);
 
-	if (use_delta_islands)
+	if (use_delta_islands) {
 		max_layers = compute_pack_layers(&to_pack);
+		free_island_marks();
+	}
 
 	ALLOC_ARRAY(wo, to_pack.nr_objects);
 	wo_end = 0;
diff --git a/delta-islands.c b/delta-islands.c
index 90c0d6958f4..8b234cb85b0 100644
--- a/delta-islands.c
+++ b/delta-islands.c
@@ -513,6 +513,20 @@ void propagate_island_marks(struct commit *commit)
 	}
 }
 
+void free_island_marks(void)
+{
+	struct island_bitmap *bitmap;
+
+	kh_foreach_value(island_marks, bitmap, {
+		if (!--bitmap->refcount)
+			free(bitmap);
+	});
+	kh_destroy_oid_map(island_marks);
+
+	/* detect use-after-free with a an address which is never valid: */
+	island_marks = (void *)-1;
+}
+
 int compute_pack_layers(struct packing_data *to_pack)
 {
 	uint32_t i;
diff --git a/delta-islands.h b/delta-islands.h
index eb0f952629f..8d1591ae28b 100644
--- a/delta-islands.h
+++ b/delta-islands.h
@@ -14,5 +14,6 @@ void resolve_tree_islands(struct repository *r,
 void load_delta_islands(struct repository *r, int progress);
 void propagate_island_marks(struct commit *commit);
 int compute_pack_layers(struct packing_data *to_pack);
+void free_island_marks(void);
 
 #endif /* DELTA_ISLANDS_H */