diff --git a/src/Grant/ClientCredentialsGrant.php b/src/Grant/ClientCredentialsGrant.php
index 9f647965..fab5932c 100644
--- a/src/Grant/ClientCredentialsGrant.php
+++ b/src/Grant/ClientCredentialsGrant.php
@@ -12,6 +12,7 @@
 namespace League\OAuth2\Server\Grant;
 
 use DateInterval;
+use League\OAuth2\Server\Exception\OAuthServerException;
 use League\OAuth2\Server\RequestEvent;
 use League\OAuth2\Server\ResponseTypes\ResponseTypeInterface;
 use Psr\Http\Message\ServerRequestInterface;
@@ -29,8 +30,18 @@ class ClientCredentialsGrant extends AbstractGrant
         ResponseTypeInterface $responseType,
         DateInterval $accessTokenTTL
     ) {
+        list($clientId) = $this->getClientCredentials($request);
+
+        $client = $this->getClientEntityOrFail($clientId, $request);
+
+        if (!$client->isConfidential()) {
+            $this->getEmitter()->emit(new RequestEvent(RequestEvent::CLIENT_AUTHENTICATION_FAILED, $request));
+            throw OAuthServerException::invalidClient($request);
+        }
+
         // Validate request
-        $client = $this->validateClient($request);
+        $this->validateClient($request);
+
         $scopes = $this->validateScopes($this->getRequestParameter('scope', $request, $this->defaultScope));
 
         // Finalize the requested scopes
diff --git a/tests/AuthorizationServerTest.php b/tests/AuthorizationServerTest.php
index bcd87b5b..870d546f 100644
--- a/tests/AuthorizationServerTest.php
+++ b/tests/AuthorizationServerTest.php
@@ -62,8 +62,11 @@ class AuthorizationServerTest extends TestCase
 
     public function testRespondToRequest()
     {
+        $client = new ClientEntity();
+        $client->setConfidential();
+
         $clientRepository = $this->getMockBuilder(ClientRepositoryInterface::class)->getMock();
-        $clientRepository->method('getClientEntity')->willReturn(new ClientEntity());
+        $clientRepository->method('getClientEntity')->willReturn($client);
 
         $scope = new ScopeEntity();
         $scopeRepositoryMock = $this->getMockBuilder(ScopeRepositoryInterface::class)->getMock();
diff --git a/tests/Grant/ClientCredentialsGrantTest.php b/tests/Grant/ClientCredentialsGrantTest.php
index 54be52fd..18e85bb2 100644
--- a/tests/Grant/ClientCredentialsGrantTest.php
+++ b/tests/Grant/ClientCredentialsGrantTest.php
@@ -29,6 +29,7 @@ class ClientCredentialsGrantTest extends TestCase
     public function testRespondToRequest()
     {
         $client = new ClientEntity();
+        $client->setConfidential();
         $clientRepositoryMock = $this->getMockBuilder(ClientRepositoryInterface::class)->getMock();
         $clientRepositoryMock->method('getClientEntity')->willReturn($client);
 
diff --git a/tests/Middleware/AuthorizationServerMiddlewareTest.php b/tests/Middleware/AuthorizationServerMiddlewareTest.php
index c8ed7d1a..e861c7c4 100644
--- a/tests/Middleware/AuthorizationServerMiddlewareTest.php
+++ b/tests/Middleware/AuthorizationServerMiddlewareTest.php
@@ -24,8 +24,11 @@ class AuthorizationServerMiddlewareTest extends TestCase
 
     public function testValidResponse()
     {
+        $client = new ClientEntity();
+        $client->setConfidential();
+
         $clientRepository = $this->getMockBuilder(ClientRepositoryInterface::class)->getMock();
-        $clientRepository->method('getClientEntity')->willReturn(new ClientEntity());
+        $clientRepository->method('getClientEntity')->willReturn($client);
 
         $scopeEntity = new ScopeEntity;
         $scopeRepositoryMock = $this->getMockBuilder(ScopeRepositoryInterface::class)->getMock();