diff --git a/lib/utils/index.js b/lib/utils/index.js
index 09eaea33..716cea30 100644
--- a/lib/utils/index.js
+++ b/lib/utils/index.js
@@ -83,7 +83,7 @@ export function isFunction(item?: any): boolean {
  * @param value
  * @return {boolean}
  */
-export function isString(value): Boolean {
+export function isString(value: any): boolean {
   return typeof value === 'string';
 }