diff --git a/composer.json b/composer.json
index b5217ce..0abd128 100644
--- a/composer.json
+++ b/composer.json
@@ -21,7 +21,7 @@
     "vlucas/phpdotenv": "^5.6.1"
   },
   "require-dev": {
-    "phpunit/phpunit": "^12.0.0",
+    "phpunit/phpunit": "^9.6.21",
     "squizlabs/php_codesniffer": "*"
   },
   "autoload": {
diff --git a/src/SDK/API.php b/src/SDK/API.php
index 8f616b4..f1b1b94 100644
--- a/src/SDK/API.php
+++ b/src/SDK/API.php
@@ -15,7 +15,7 @@
 
 class API
 {
-    private const RETRYABLE_STATUS_CODES = [503, 521, 522, 524, 530];
+    private const RETRYABLE_STATUS_CODES = [503, 520, 521, 522, 524, 530];
 
     private $httpClient;
     private $projectId;
@@ -244,7 +244,7 @@ public function generateJwtResponse(array $responseBody, ?string $refreshToken =
 
     /**
      * Executes an HTTP request callable, retrying on transient status codes
-     * (503, 521, 522, 524, 530) with delays of 100ms, 5s, 5s.
+     * (503, 520, 521, 522, 524, 530) with delays of 100ms, 5s, 5s.
      * Non-retryable RequestExceptions are re-thrown immediately.
      *
      * @param  callable $requestFn Zero-argument callable that performs the Guzzle request.
diff --git a/src/tests/APIRetryTest.php b/src/tests/APIRetryTest.php
index ef367e0..e370835 100644
--- a/src/tests/APIRetryTest.php
+++ b/src/tests/APIRetryTest.php
@@ -62,7 +62,7 @@ public function testRetriesOnRetryableStatusCodeAndSucceedsOnSecondAttempt(int $
 
     public static function retryableStatusCodeProvider(): array
     {
-        return [[503], [521], [522], [524], [530]];
+        return [[503], [520], [521], [522], [524], [530]];
     }
 
     public function testRetriesUpToThreeTimesAndThrowsOnExhaustion(): void