paul121
--- a/tests/src/Kernel/ClientCredentialsTest.php
+++ b/tests/src/Kernel/ClientCredentialsTest.php
+      'no_client_secret' => [
+        'NULL',
+        'invalid_client',
+        401,
+      ],
+    ];
+  }
+
+  /**
+   * Test public client with ClientCredentials grant.
+   *
+   * The client credentials grant cannot be used with a public client.
+   *
+   * @dataProvider publicClientCredentialsGrantProvider
+   */
+  public function testPublicClientCredentialsGrant(string $secret, string $error, int $code): void {
--- a/tests/src/Kernel/ClientCredentialsTest.php
+++ b/tests/src/Kernel/ClientCredentialsTest.php
+   */
+  public function testPublicClientCredentialsGrant(string $secret, string $error, int $code): void {
+    $this->client->set('secret', NULL)->save();
+    $parameters = [
+      'grant_type' => 'client_credentials',
+      'client_id' => $this->client->getClientId(),
+      'scope' => $this->scope,
+    ];
+    if ($secret) {
+      $parameters['client_secret'] = $secret;
+    }
+    $request = Request::create($this->url->toString(), 'POST', $parameters);
+    $response = $this->httpKernel->handle($request);
+    $parsed_response = Json::decode((string) $response->getContent());
+    $this->assertSame($code, $response->getStatusCode(), sprintf('Correct status code %d', $code));
+    $this->assertSame($error, $parsed_response['error'], sprintf('Correct error code %s', $error));