diff --git a/src/lib/util/assertString.js b/src/lib/util/assertString.js
index 6a4737b4b..e4fac8854 100644
--- a/src/lib/util/assertString.js
+++ b/src/lib/util/assertString.js
@@ -1,5 +1,7 @@
 export default function assertString(input) {
-  if (typeof input !== 'string') {
+  const isString = (typeof input === 'string' || input instanceof String);
+
+  if (!isString) {
     throw new TypeError('This library (validator.js) validates strings only');
   }
 }