diff --git a/src/Grant/AuthCodeGrant.php b/src/Grant/AuthCodeGrant.php
index 6f2b6ff8..18720f45 100644
--- a/src/Grant/AuthCodeGrant.php
+++ b/src/Grant/AuthCodeGrant.php
@@ -144,7 +144,7 @@ class AuthCodeGrant extends AbstractAuthorizeGrant
                 case 'S256':
                     if (
                         hash_equals(
-                            hash('sha256', strtr(rtrim(base64_encode($codeVerifier), '='), '+/', '-_')),
+                            strtr(rtrim(base64_encode(hash('sha256', $codeVerifier, true)), '='), '+/', '-_'),
                             $authCodePayload->code_challenge
                         ) === false
                     ) {
diff --git a/tests/Grant/AuthCodeGrantTest.php b/tests/Grant/AuthCodeGrantTest.php
index 6d0e801e..97206a76 100644
--- a/tests/Grant/AuthCodeGrantTest.php
+++ b/tests/Grant/AuthCodeGrantTest.php
@@ -744,6 +744,10 @@ class AuthCodeGrantTest extends TestCase
         $grant->setRefreshTokenRepository($refreshTokenRepositoryMock);
         $grant->setEncryptionKey($this->cryptStub->getKey());
 
+        // [RFC 7636] Appendix B.  Example for the S256 code_challenge_method
+        $codeVerifier = 'dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk';
+        $codeChallenge = 'E9Melhoa2OwvFrEMTJguCHaoeK1t8URWbuGJSstw-cM';
+
         $request = new ServerRequest(
             [],
             [],
@@ -757,7 +761,7 @@ class AuthCodeGrantTest extends TestCase
                 'grant_type'    => 'authorization_code',
                 'client_id'     => 'foo',
                 'redirect_uri'  => 'http://foo/bar',
-                'code_verifier' => 'foobar',
+                'code_verifier' => $codeVerifier,
                 'code'          => $this->cryptStub->doEncrypt(
                     json_encode(
                         [
@@ -767,7 +771,7 @@ class AuthCodeGrantTest extends TestCase
                             'user_id'               => 123,
                             'scopes'                => ['foo'],
                             'redirect_uri'          => 'http://foo/bar',
-                            'code_challenge'        => hash('sha256', strtr(rtrim(base64_encode('foobar'), '='), '+/', '-_')),
+                            'code_challenge'        => $codeChallenge,
                             'code_challenge_method' => 'S256',
                         ]
                     )