diff --git a/delta-islands.c b/delta-islands.c
index 8b234cb85b..afdec0a878 100644
--- a/delta-islands.c
+++ b/delta-islands.c
@@ -517,11 +517,13 @@ 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);
+	if (island_marks) {
+		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;
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index b5f9b10922..499d5d4c78 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -1015,4 +1015,20 @@ test_expect_success 'complains when run outside of a repository' '
 	grep "not a git repository" err
 '
 
+test_expect_success 'repack with delta islands' '
+	git init repo &&
+	test_when_finished "rm -fr repo" &&
+	(
+		cd repo &&
+
+		test_commit first &&
+		git repack &&
+		test_commit second &&
+		git repack &&
+
+		git multi-pack-index write &&
+		git -c repack.useDeltaIslands=true multi-pack-index repack
+	)
+'
+
 test_done