diff --git a/lib/Core/Util.php b/lib/Core/Util.php
index fa20859f..fdd9be1f 100644
--- a/lib/Core/Util.php
+++ b/lib/Core/Util.php
@@ -23,7 +23,7 @@ public static function utf8($value)
      *
      * @return string A querystring, essentially.
      */
-    public static function encodeQueryParams($arr, $prefix = null)
+    public static function encodeQueryParams($arr, string|null $prefix = null)
     {
         if (!is_array($arr)) {
             return $arr;
diff --git a/lib/RetryMiddlewareFactory.php b/lib/RetryMiddlewareFactory.php
index 3ed339b8..8f675298 100644
--- a/lib/RetryMiddlewareFactory.php
+++ b/lib/RetryMiddlewareFactory.php
@@ -32,8 +32,8 @@ private static function buildRetryDecider()
         return function (
             $retries,
             \GuzzleHttp\Psr7\Request $request,
-            \GuzzleHttp\Psr7\Response $response = null,
-            \GuzzleHttp\Exception\TransferException $exception = null
+            \GuzzleHttp\Psr7\Response|null $response = null,
+            \GuzzleHttp\Exception\TransferException|null $exception = null
         ) {
             if ($retries >= self::MAX_AUTOMATIC_TIMEOUT_RETRIES) {
                 return false;
@@ -78,7 +78,7 @@ private static function buildRetryDelay()
      *
      * @return boolean
      */
-    private static function isConnectionError(\GuzzleHttp\Exception\TransferException $exception = null)
+    private static function isConnectionError(\GuzzleHttp\Exception\TransferException|null $exception = null)
     {
         return $exception instanceof \GuzzleHttp\Exception\ConnectException;
     }
@@ -90,7 +90,7 @@ private static function isConnectionError(\GuzzleHttp\Exception\TransferExceptio
      *
      * @return boolean
      */
-    private static function isRetryableServerError(\GuzzleHttp\Psr7\Response $response = null)
+    private static function isRetryableServerError(\GuzzleHttp\Psr7\Response|null $response = null)
     {
         if ($response) {
             $statusCode = $response->getStatusCode();