diff --git a/src/Repositories/ScopeRepositoryInterface.php b/src/Repositories/ScopeRepositoryInterface.php
index 95bfdbb9a..e5ae7c716 100644
--- a/src/Repositories/ScopeRepositoryInterface.php
+++ b/src/Repositories/ScopeRepositoryInterface.php
@@ -39,7 +39,7 @@ public function finalizeScopes(
         array $scopes,
         string $grantType,
         ClientEntityInterface $clientEntity,
-        string|int|null $userIdentifier = null,
+        string|null $userIdentifier = null,
         ?string $authCodeId = null
     ): array;
 }
diff --git a/tests/Grant/AuthCodeGrantTest.php b/tests/Grant/AuthCodeGrantTest.php
index 867c785ec..6a6842661 100644
--- a/tests/Grant/AuthCodeGrantTest.php
+++ b/tests/Grant/AuthCodeGrantTest.php
@@ -1013,7 +1013,7 @@ public function testPKCEDowngradeBlocked(): void
                             'auth_code_id'          => uniqid(),
                             'expire_time'           => time() + 3600,
                             'client_id'             => 'foo',
-                            'user_id'               => 123,
+                            'user_id'               => '123',
                             'scopes'                => ['foo'],
                             'redirect_uri'          => self::REDIRECT_URI,
                         ],
@@ -1561,7 +1561,7 @@ public function testRespondToAccessTokenRequestBadCodeVerifierPlain(): void
                         'auth_code_id'          => uniqid(),
                         'expire_time'           => time() + 3600,
                         'client_id'             => 'foo',
-                        'user_id'               => 123,
+                        'user_id'               => '123',
                         'scopes'                => ['foo'],
                         'redirect_uri'          => self::REDIRECT_URI,
                         'code_challenge'        => 'foobar',
@@ -1636,7 +1636,7 @@ public function testRespondToAccessTokenRequestBadCodeVerifierS256(): void
                         'auth_code_id'          => uniqid(),
                         'expire_time'           => time() + 3600,
                         'client_id'             => 'foo',
-                        'user_id'               => 123,
+                        'user_id'               => '123',
                         'scopes'                => ['foo'],
                         'redirect_uri'          => self::REDIRECT_URI,
                         'code_challenge'        => 'foobar',
@@ -1711,7 +1711,7 @@ public function testRespondToAccessTokenRequestMalformedCodeVerifierS256WithInva
                         'auth_code_id'          => uniqid(),
                         'expire_time'           => time() + 3600,
                         'client_id'             => 'foo',
-                        'user_id'               => 123,
+                        'user_id'               => '123',
                         'scopes'                => ['foo'],
                         'redirect_uri'          => self::REDIRECT_URI,
                         'code_challenge'        => self::CODE_CHALLENGE,
@@ -1786,7 +1786,7 @@ public function testRespondToAccessTokenRequestMalformedCodeVerifierS256WithInva
                         'auth_code_id'          => uniqid(),
                         'expire_time'           => time() + 3600,
                         'client_id'             => 'foo',
-                        'user_id'               => 123,
+                        'user_id'               => '123',
                         'scopes'                => ['foo'],
                         'redirect_uri'          => self::REDIRECT_URI,
                         'code_challenge'        => 'R7T1y1HPNFvs1WDCrx4lfoBS6KD2c71pr8OHvULjvv8',
@@ -1860,7 +1860,7 @@ public function testRespondToAccessTokenRequestMissingCodeVerifier(): void
                         'auth_code_id'          => uniqid(),
                         'expire_time'           => time() + 3600,
                         'client_id'             => 'foo',
-                        'user_id'               => 123,
+                        'user_id'               => '123',
                         'scopes'                => ['foo'],
                         'redirect_uri'          => self::REDIRECT_URI,
                         'code_challenge'        => 'foobar',