diff --git a/examples/src/Repositories/AccessTokenRepository.php b/examples/src/Repositories/AccessTokenRepository.php
index affd9be8a..1e6da86a4 100644
--- a/examples/src/Repositories/AccessTokenRepository.php
+++ b/examples/src/Repositories/AccessTokenRepository.php
@@ -49,11 +49,16 @@ public function isAccessTokenRevoked($tokenId): bool
     public function getNewToken(ClientEntityInterface $clientEntity, array $scopes, $userIdentifier = null): AccessTokenEntityInterface
     {
         $accessToken = new AccessTokenEntity();
+
         $accessToken->setClient($clientEntity);
+
         foreach ($scopes as $scope) {
             $accessToken->addScope($scope);
         }
-        $accessToken->setUserIdentifier($userIdentifier);
+
+        if ($userIdentifier !== null) {
+            $accessToken->setUserIdentifier($userIdentifier);
+        }
 
         return $accessToken;
     }
diff --git a/src/Entities/Traits/AccessTokenTrait.php b/src/Entities/Traits/AccessTokenTrait.php
index 015002069..99f3cbc9e 100644
--- a/src/Entities/Traits/AccessTokenTrait.php
+++ b/src/Entities/Traits/AccessTokenTrait.php
@@ -59,12 +59,6 @@ public function initJwtConfiguration(): void
      */
     private function convertToJWT(): Token
     {
-        $userIdentifier = $this->getUserIdentifier();
-
-        if ($userIdentifier === null) {
-            throw new RuntimeException('JWT access tokens MUST contain a subject identifier');
-        }
-
         $this->initJwtConfiguration();
 
         return $this->jwtConfiguration->builder()
@@ -73,7 +67,7 @@ private function convertToJWT(): Token
             ->issuedAt(new DateTimeImmutable())
             ->canOnlyBeUsedAfter(new DateTimeImmutable())
             ->expiresAt($this->getExpiryDateTime())
-            ->relatedTo($userIdentifier)
+            ->relatedTo($this->getSubjectIdentifier())
             ->withClaim('scopes', $this->getScopes())
             ->getToken($this->jwtConfiguration->signer(), $this->jwtConfiguration->signingKey());
     }
@@ -104,4 +98,15 @@ abstract public function getScopes(): array;
      * @return non-empty-string
      */
     abstract public function getIdentifier(): string;
+
+    private function getSubjectIdentifier(): string
+    {
+        $subjectId = $this->getUserIdentifier() ?? $this->getClient()->getIdentifier();
+
+        if ($subjectId === null) {
+            throw new RuntimeException('JWT access tokens MUST contain a subject identifier');
+        }
+
+        return $subjectId;
+    }
 }
diff --git a/src/ResponseTypes/BearerTokenResponse.php b/src/ResponseTypes/BearerTokenResponse.php
index 0c6107b9e..24ddc8c8e 100644
--- a/src/ResponseTypes/BearerTokenResponse.php
+++ b/src/ResponseTypes/BearerTokenResponse.php
@@ -34,21 +34,23 @@ public function generateHttpResponse(ResponseInterface $response): ResponseInter
             'access_token' => $this->accessToken->toString(),
         ];
 
-        $refreshTokenPayload = json_encode([
-                'client_id'        => $this->accessToken->getClient()->getIdentifier(),
-                'refresh_token_id' => $this->refreshToken->getIdentifier(),
-                'access_token_id'  => $this->accessToken->getIdentifier(),
-                'scopes'           => $this->accessToken->getScopes(),
-                'user_id'          => $this->accessToken->getUserIdentifier(),
-                'expire_time'      => $this->refreshToken->getExpiryDateTime()->getTimestamp(),
-        ]);
-
-        if ($refreshTokenPayload === false) {
-            throw new LogicException('Error encountered JSON encoding the refresh token payload');
+        if (isset($this->refreshToken)) {
+            $refreshTokenPayload = json_encode([
+                    'client_id'        => $this->accessToken->getClient()->getIdentifier(),
+                    'refresh_token_id' => $this->refreshToken->getIdentifier(),
+                    'access_token_id'  => $this->accessToken->getIdentifier(),
+                    'scopes'           => $this->accessToken->getScopes(),
+                    'user_id'          => $this->accessToken->getUserIdentifier(),
+                    'expire_time'      => $this->refreshToken->getExpiryDateTime()->getTimestamp(),
+            ]);
+
+            if ($refreshTokenPayload === false) {
+                throw new LogicException('Error encountered JSON encoding the refresh token payload');
+            }
+
+            $responseParams['refresh_token'] = $this->encrypt($refreshTokenPayload);
         }
-
-        $responseParams['refresh_token'] = $this->encrypt($refreshTokenPayload);
-
+        
         $responseParams = json_encode(array_merge($this->getExtraParams($this->accessToken), $responseParams));
 
         if ($responseParams === false) {