diff -u b/src/AuthenticationPluginInterface.php b/src/AuthenticationPluginInterface.php
--- b/src/AuthenticationPluginInterface.php
+++ b/src/AuthenticationPluginInterface.php
@@ -18,6 +18,8 @@
    * @return array
    *   Options (such as Authentication headers) to be added to the request.
+   *
+   * @link http://docs.guzzlephp.org/en/latest/request-options.html
    */
-  public function authenticate();
+  public function getAuthenticationOptions();
 
 }
diff -u b/src/Plugin/migrate_plus/authentication/Basic.php b/src/Plugin/migrate_plus/authentication/Basic.php
--- b/src/Plugin/migrate_plus/authentication/Basic.php
+++ b/src/Plugin/migrate_plus/authentication/Basic.php
@@ -18,7 +18,7 @@
   /**
    * {@inheritdoc}
    */
-  public function authenticate() {
+  public function getAuthenticationOptions() {
     return [
       'auth' => [
         $this->configuration['username'],
diff -u b/src/Plugin/migrate_plus/authentication/Digest.php b/src/Plugin/migrate_plus/authentication/Digest.php
--- b/src/Plugin/migrate_plus/authentication/Digest.php
+++ b/src/Plugin/migrate_plus/authentication/Digest.php
@@ -18,7 +18,7 @@
   /**
    * {@inheritdoc}
    */
-  public function authenticate() {
+  public function getAuthenticationOptions() {
     return [
       'auth' => [
         $this->configuration['username'],
diff -u b/src/Plugin/migrate_plus/authentication/OAuth2.php b/src/Plugin/migrate_plus/authentication/OAuth2.php
--- b/src/Plugin/migrate_plus/authentication/OAuth2.php
+++ b/src/Plugin/migrate_plus/authentication/OAuth2.php
@@ -3,10 +3,10 @@
 namespace Drupal\migrate_plus\Plugin\migrate_plus\authentication;
 
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\migrate\MigrateException;
 use Drupal\migrate_plus\AuthenticationPluginBase;
 use GuzzleHttp\Client;
 use GuzzleHttp\HandlerStack;
-use Sainsburys\Guzzle\Oauth2\AccessToken;
 use Sainsburys\Guzzle\Oauth2\GrantType\AuthorizationCode;
 use Sainsburys\Guzzle\Oauth2\GrantType\ClientCredentials;
 use Sainsburys\Guzzle\Oauth2\GrantType\JwtBearer;
@@ -16,6 +16,8 @@
 
 /**
  * Provides OAuth2 authentication for the HTTP resource.
+ * 
+ * @link https://packagist.org/packages/sainsburys/guzzle-oauth2-plugin
  *
  * @Authentication(
  *   id = "oauth2",
@@ -27,7 +29,7 @@
   /**
    * {@inheritdoc}
    */
-  public function authenticate() {
+  public function getAuthenticationOptions() {
     $handlerStack = HandlerStack::create();
     $client = new Client([
       'handler'=> $handlerStack,
@@ -51,6 +53,9 @@
       case 'refresh_token':
         $grant_type = new RefreshToken($client, $this->configuration);
         break;
+      default:
+        throw new MigrateException("Unrecognized grant_type {$this->configuration['grant_type']}.");
+        break;
     }
     $middleware = new OAuthMiddleware($client, $grant_type);
 
diff -u b/src/Plugin/migrate_plus/data_fetcher/Http.php b/src/Plugin/migrate_plus/data_fetcher/Http.php
--- b/src/Plugin/migrate_plus/data_fetcher/Http.php
+++ b/src/Plugin/migrate_plus/data_fetcher/Http.php
@@ -80,7 +80,7 @@
     try {
       $options = ['headers' => $this->getRequestHeaders()];
       if (!empty($this->configuration['authentication'])) {
-        $options = array_merge($options, $this->getAuthenticationPlugin()->authenticate());
+        $options = array_merge($options, $this->getAuthenticationPlugin()->getAuthenticationOptions());
       }
       $response = $this->httpClient->get($url, $options);
       if (empty($response)) {