diff --git a/src/acdhOeaw/arche/core/BadRequestException.php b/src/acdhOeaw/arche/core/BadRequestException.php
index 22b2aa2..c1f9537 100644
--- a/src/acdhOeaw/arche/core/BadRequestException.php
+++ b/src/acdhOeaw/arche/core/BadRequestException.php
@@ -33,7 +33,7 @@
 class BadRequestException extends RepoException {
 
     public function __construct(string $message = "", int $code = 400,
-                                \Throwable $previous = null) {
+                                ?\Throwable $previous = null) {
         parent::__construct($message, $code, $previous);
     }
 }
diff --git a/src/acdhOeaw/arche/core/ConflictException.php b/src/acdhOeaw/arche/core/ConflictException.php
index 1d7b68c..68118fd 100644
--- a/src/acdhOeaw/arche/core/ConflictException.php
+++ b/src/acdhOeaw/arche/core/ConflictException.php
@@ -33,7 +33,7 @@
 class ConflictException extends BadRequestException {
 
     public function __construct(string $message = "", int $code = 409,
-                                \Throwable $previous = null) {
+                                ?\Throwable $previous = null) {
         parent::__construct($message, $code, $previous);
     }
 }
diff --git a/src/acdhOeaw/arche/core/DuplicatedKeyException.php b/src/acdhOeaw/arche/core/DuplicatedKeyException.php
index fe2a165..2ab8b23 100644
--- a/src/acdhOeaw/arche/core/DuplicatedKeyException.php
+++ b/src/acdhOeaw/arche/core/DuplicatedKeyException.php
@@ -33,7 +33,7 @@
 class DuplicatedKeyException extends RepoException {
 
     public function __construct(string $message = "Duplicate key value violates unique constraint", int $code = 409,
-                                \Throwable $previous = null) {
+                                ?\Throwable $previous = null) {
         parent::__construct($message, $code, $previous);
     }
 }
diff --git a/src/acdhOeaw/arche/core/Metadata.php b/src/acdhOeaw/arche/core/Metadata.php
index 65f248c..7bb08be 100644
--- a/src/acdhOeaw/arche/core/Metadata.php
+++ b/src/acdhOeaw/arche/core/Metadata.php
@@ -145,7 +145,7 @@ public function update(DatasetNode $newMeta, array $preserve = []): void {
         $this->graph->add($newMeta->map(fn($x) => $x->withSubject($node))->withNode($node));
     }
 
-    public function loadFromRequest(string $resUri = null): int {
+    public function loadFromRequest(?string $resUri = null): int {
         $format      = filter_input(INPUT_SERVER, 'CONTENT_TYPE');
         $length      = (int) filter_input(INPUT_SERVER, 'CONTENT_LENGTH');
         $node        = DF::namedNode($this->getUri());
diff --git a/tests/TestBase.php b/tests/TestBase.php
index 7e770b4..a1775e7 100644
--- a/tests/TestBase.php
+++ b/tests/TestBase.php
@@ -255,7 +255,7 @@ protected function updateResource(DatasetNode $meta, ?int $txId = null,
         return $resp;
     }
 
-    protected function deleteResource(string $location, int $txId = null): bool {
+    protected function deleteResource(string $location, ?int $txId = null): bool {
         $extTx = $txId !== null;
         if (!$extTx) {
             $txId = $this->beginTransaction();
@@ -276,7 +276,7 @@ protected function deleteResource(string $location, int $txId = null): bool {
      * @param int $txId
      * @return array<string, mixed>
      */
-    protected function getHeaders(int $txId = null): array {
+    protected function getHeaders(?int $txId = null): array {
         return [
             self::$config->rest->headers->transactionId => $txId,
             'Eppn'                                      => 'admin',