diff --git a/test/OAuth/OAuthTest.php b/test/OAuth/OAuthTest.php
index e141b6b7..14e1a7be 100644
--- a/test/OAuth/OAuthTest.php
+++ b/test/OAuth/OAuthTest.php
@@ -14,6 +14,7 @@
 namespace PHPMailer\Test\OAuth;
 
 use PHPMailer\PHPMailer\OAuth;
+use PHPMailer\PHPMailer\OAuthTokenProvider;
 use PHPMailer\PHPMailer\PHPMailer;
 use Yoast\PHPUnitPolyfills\TestCases\TestCase;
 
@@ -28,10 +29,11 @@ final class OAuthTest extends TestCase
      * @covers PHPMailer\PHPMailer\PHPMailer::getOAuth
      * @covers PHPMailer\PHPMailer\PHPMailer::setOAuth
      * @covers PHPMailer\PHPMailer\OAuth::__construct
+     * @covers PHPMailer\PHPMailer\OAuthTokenProvider
      */
     public function testOAuth()
     {
-        $PHPMailer = new PHPMailer();
+        $PHPMailer = new PHPMailer(true);
         $reflection = new \ReflectionClass($PHPMailer);
         $property = $reflection->getProperty('oauth');
         $property->setAccessible(true);
@@ -48,6 +50,7 @@ final class OAuthTest extends TestCase
 
         $oauth = new OAuth($options);
         self::assertInstanceOf(OAuth::class, $oauth, 'Instantiation of OAuth class failed');
+        self::assertInstanceOf(OAuthTokenProvider::class, $oauth, 'Instantiation of OAuth class failed');
         $subject = $PHPMailer->setOAuth($oauth);
         self::assertNull($subject, 'setOAuth() is not a void function');
         self::assertInstanceOf(
@@ -55,5 +58,16 @@ final class OAuthTest extends TestCase
             $PHPMailer->getOAuth(),
             'Setting Oauth property to an instance of the OAuth class failed'
         );
+        $this->expectException(\TypeError::class);
+        $PHPMailer->setOAuth(new \stdClass());
+        $PHPMailer->setOAuth(new testOAuthProvider());
     }
 }
+
+class testOAuthProvider implements OAuthTokenProvider
+{
+    public function getOauth64()
+    {
+        return 'oauth';
+    }
+}
\ No newline at end of file