diff -up ./lib/Auth/Digest.php.up ./lib/Auth/Digest.php
--- ./lib/Auth/Digest.php.up	2019-08-20 14:37:51.705235293 +0200
+++ ./lib/Auth/Digest.php	2019-08-20 14:38:29.371438395 +0200
@@ -118,7 +118,7 @@ class Digest extends AbstractAuth {
      */
     function getUsername() {
 
-        return $this->digestParts['username'];
+        return $this->digestParts['username'] ?? null;
 
     }
 
@@ -127,6 +127,10 @@ class Digest extends AbstractAuth {
      */
     protected function validate() : bool {
 
+        if (!is_array($this->digestParts)) {
+            return false;
+        }
+
         $A2 = $this->request->getMethod() . ':' . $this->digestParts['uri'];
 
         if ($this->digestParts['qop'] == 'auth-int') {