diff --git a/lib/validate.js b/lib/validate.js
index 1baa31c9..446b1731 100644
--- a/lib/validate.js
+++ b/lib/validate.js
@@ -159,6 +159,7 @@ const hexOrBase64Validator = string().test({
   name: 'hex-or-base64',
   message: 'invalid encoding',
   test: (val) => {
+    if (typeof val === 'undefined') return true
     try {
       ensureB64(val)
       return true