diff --git a/src/Auth/Source/X509userCert.php b/src/Auth/Source/X509userCert.php
index a4cff35..d9f01f6 100644
--- a/src/Auth/Source/X509userCert.php
+++ b/src/Auth/Source/X509userCert.php
@@ -277,8 +277,8 @@ public function findUserByAttribute(string $attr, string $value): ?Entry
     {
         $searchBase = $this->ldapConfig->getArray('search.base');
 
-        $searchUsername = $this->ldapConfig->getString('search.username');
-        Assert::notWhitespaceOnly($searchUsername);
+        $searchUsername = $this->ldapConfig->getOptionalString('search.username', null);
+        Assert::nullOrnotWhitespaceOnly($searchUsername);
 
         $searchPassword = $this->ldapConfig->getOptionalString('search.password', null);
         Assert::nullOrnotWhitespaceOnly($searchPassword);