diff --git a/lib/API/Endpoint.php b/lib/API/Endpoint.php
index 98b406ff51..4cc419c481 100644
--- a/lib/API/Endpoint.php
+++ b/lib/API/Endpoint.php
@@ -17,7 +17,7 @@ abstract class Endpoint {
   ) {
     if(empty($errors)) {
       $errors = array(
-        'unknown' => __('An unknown error occurred.')
+        Error::UNKNOWN  => __('An unknown error occurred.')
       );
     }
     return new ErrorResponse($errors, $meta, $status);
@@ -26,7 +26,7 @@ abstract class Endpoint {
   function badRequest($errors = array(), $meta = array()) {
     if(empty($errors)) {
       $errors = array(
-        'bad_request' => __('Invalid request parameters.')
+        Error::BAD_REQUEST => __('Invalid request parameters.')
       );
     }
     return new ErrorResponse($errors, $meta, Response::STATUS_BAD_REQUEST);
diff --git a/lib/API/Endpoints/Settings.php b/lib/API/Endpoints/Settings.php
index dcf664e93e..6c8dcaa335 100644
--- a/lib/API/Endpoints/Settings.php
+++ b/lib/API/Endpoints/Settings.php
@@ -1,6 +1,7 @@
 <?php
 namespace MailPoet\API\Endpoints;
 use \MailPoet\API\Endpoint as APIEndpoint;
+use \MailPoet\API\Error as APIError;
 use \MailPoet\Models\Setting;
 
 if(!defined('ABSPATH')) exit;
@@ -10,18 +11,20 @@ class Settings extends APIEndpoint {
   }
 
   function get() {
-    $settings = Setting::getAll();
-    return $this->successResponse($settings);
+    return $this->successResponse(Setting::getAll());
   }
 
   function set($settings = array()) {
     if(empty($settings)) {
-      return $this->badRequest();
+      return $this->badRequest(array(
+        APIError::BAD_REQUEST =>
+          __("You have not specified any settings to be saved.")
+      ));
     } else {
       foreach($settings as $name => $value) {
         Setting::setValue($name, $value);
       }
-      return $this->successResponse();
+      return $this->successResponse(Setting::getAll());
     }
   }
 }
diff --git a/lib/API/Endpoints/Setup.php b/lib/API/Endpoints/Setup.php
index 90651784c0..65f03403d0 100644
--- a/lib/API/Endpoints/Setup.php
+++ b/lib/API/Endpoints/Setup.php
@@ -17,7 +17,7 @@ class Setup extends APIEndpoint {
       return $this->successResponse();
     } catch(\Exception $e) {
       return $this->errorResponse(array(
-        $e->getMessage()
+        $e->getCode() => $e->getMessage()
       ));
     }
   }
diff --git a/lib/API/Error.php b/lib/API/Error.php
new file mode 100644
index 0000000000..c151f92364
--- /dev/null
+++ b/lib/API/Error.php
@@ -0,0 +1,11 @@
+<?php
+namespace MailPoet\API;
+
+if(!defined('ABSPATH')) exit;
+
+final class Error {
+  const UNKNOWN = 'unknown';
+  const BAD_REQUEST = 'bad_request';
+
+  private function __construct() {}
+}
\ No newline at end of file
diff --git a/tests/unit/API/SettingsTest.php b/tests/unit/API/SettingsTest.php
index c5b5c068f7..6bb1bfaa62 100644
--- a/tests/unit/API/SettingsTest.php
+++ b/tests/unit/API/SettingsTest.php
@@ -1,5 +1,6 @@
 <?php
-use \MailPoet\API\Response;
+use \MailPoet\API\Response as APIResponse;
+use \MailPoet\API\Error as APIError;
 use \MailPoet\API\Endpoints\Settings;
 use \MailPoet\Models\Setting;
 
@@ -12,14 +13,14 @@ class SettingsTest extends MailPoetTest {
     $router = new Settings();
 
     $response = $router->get();
-    expect($response->status)->equals(Response::STATUS_OK);
+    expect($response->status)->equals(APIResponse::STATUS_OK);
 
     expect($response->data)->notEmpty();
     expect($response->data['some']['setting']['key'])->true();
 
     Setting::deleteMany();
     $response = $router->get();
-    expect($response->status)->equals(Response::STATUS_OK);
+    expect($response->status)->equals(APIResponse::STATUS_OK);
     expect($response->data)->equals(Setting::getDefaults());
   }
 
@@ -36,14 +37,14 @@ class SettingsTest extends MailPoetTest {
     $router = new Settings();
 
     $response = $router->set(/* missing data */);
-    expect($response->errors[0]['error'])->equals('bad_request');
-    expect($response->status)->equals(Response::STATUS_BAD_REQUEST);
+    expect($response->errors[0]['error'])->equals(APIError::BAD_REQUEST);
+    expect($response->status)->equals(APIResponse::STATUS_BAD_REQUEST);
 
     $response = $router->set($new_settings);
-    expect($response->status)->equals(Response::STATUS_OK);
+    expect($response->status)->equals(APIResponse::STATUS_OK);
 
     $response = $router->get();
-    expect($response->status)->equals(Response::STATUS_OK);
+    expect($response->status)->equals(APIResponse::STATUS_OK);
     expect($response->data['some']['setting'])->hasntKey('key');
     expect($response->data['some']['setting']['new_key'])->true();
     expect($response->data['some']['new_setting'])->true();
diff --git a/tests/unit/API/SetupTest.php b/tests/unit/API/SetupTest.php
index 9b45ca14c3..0f4a9e6092 100644
--- a/tests/unit/API/SetupTest.php
+++ b/tests/unit/API/SetupTest.php
@@ -1,5 +1,5 @@
 <?php
-use \MailPoet\API\Response;
+use \MailPoet\API\Response as APIResponse;
 use \MailPoet\API\Endpoints\Setup;
 use \MailPoet\Models\Setting;
 
@@ -11,7 +11,7 @@ class SetupTest extends MailPoetTest {
   function testItCanReinstall() {
     $router = new Setup();
     $response = $router->reset();
-    expect($response->status)->equals(Response::STATUS_OK);
+    expect($response->status)->equals(APIResponse::STATUS_OK);
 
     $signup_confirmation = Setting::getValue('signup_confirmation.enabled');
     expect($signup_confirmation)->true();