diff --git a/index.js b/index.js
index d92103f..b4af5b7 100644
--- a/index.js
+++ b/index.js
@@ -1,3 +1,7 @@
+if ('toJSON' in Error.prototype) {
+  return;
+}
+
 Object.defineProperty(Error.prototype, 'toJSON', {
     value: function () {
         var alt = {};