diff --git a/package.json b/package.json
index b77ae3c..fdbc6fc 100644
--- a/package.json
+++ b/package.json
@@ -33,5 +33,8 @@
     "LICENSE",
     "index.js",
     "index.d.ts"
-  ]
-}
\ No newline at end of file
+  ],
+  "engines": {
+    "node": ">=10"
+  }
+}
diff --git a/test/index.js b/test/index.js
index 78c2cab..d6e3b3d 100644
--- a/test/index.js
+++ b/test/index.js
@@ -34,11 +34,15 @@ test ('try_parse_json should fail', (t) => {
 });
 
 test ('copy object', (t) => {
-  const obj = { foo: 'bar' };
+  const obj = { foo: 'bar', bar: { foo: 'baz' } };
   const copy = util.copy_object (obj);
+  t.deepEqual (obj, copy);
   copy.foo = 'baz';
+  copy.bar.foo = 'bar';
   t.is (copy.foo, 'baz');
+  t.is (copy.bar.foo, 'bar');
   t.is (obj.foo, 'bar');
+  t.is (obj.bar.foo, 'baz');
 });
 
 test ('run regex', (t) => {