diff --git a/lib/validators.js b/lib/validators.js
index 85581641..00dc943a 100644
--- a/lib/validators.js
+++ b/lib/validators.js
@@ -28,6 +28,9 @@ SOFTWARE.
 "use strict";
 
 /* Validation functions copied from check-types package - https://www.npmjs.com/package/check-types */
+
+const toString = Object.prototype.toString;
+
 function isFunction(data) {
   return typeof data === "function";
 }