diff --git a/base64.ts b/base64.ts
index 845d1ee..7670f3c 100644
--- a/base64.ts
+++ b/base64.ts
@@ -215,7 +215,7 @@ const decode = (src: string) => _decode(_unURI(src));
  * check if a value is a valid Base64 string
  * @param {String} src a value to check
   */
-const isValid = (src: any) => {
+const isValid = (src: unknown) => {
     if (typeof src !== 'string') return false;
     const s = src.replace(/\s+/g, '').replace(/={0,2}$/, '');
     return !/[^\s0-9a-zA-Z\+/]/.test(s) || !/[^\s0-9a-zA-Z\-_]/.test(s);