diff --git a/core/modules/rest/src/Plugin/rest/resource/UserLoginResource.php b/core/modules/rest/src/Plugin/rest/resource/UserLoginResource.php
index daf3640..4966ed1 100644
--- a/core/modules/rest/src/Plugin/rest/resource/UserLoginResource.php
+++ b/core/modules/rest/src/Plugin/rest/resource/UserLoginResource.php
@@ -111,7 +111,7 @@ public function post(array $operation = array()) {
       }
     }
     else {
-      throw new BadRequestHttpException('No op found.');
+      throw new BadRequestHttpException('No op found. Use: status, login, logout.');
     }
   }
 
diff --git a/core/modules/rest/src/Tests/UserTest.php b/core/modules/rest/src/Tests/UserTest.php
index 66505e6..4c2d48a 100644
--- a/core/modules/rest/src/Tests/UserTest.php
+++ b/core/modules/rest/src/Tests/UserTest.php
@@ -41,7 +41,7 @@ public function testLogin() {
 
     $payload = array();
     $this->httpRequest('user_login', 'POST', json_encode($payload), $this->defaultMimeType, $basic_auth);
-    $this->assertResponseAndText(400, 'No op found.');
+    $this->assertResponseAndText(400, 'No op found. Use: status, login, logout.');
 
     $payload = $this->getPayload('garbage');
     $this->httpRequest('user_login', 'POST', json_encode($payload), $this->defaultMimeType, $basic_auth);
diff --git a/core/modules/rest/tests/src/Unit/UserLoginResourceTest.php b/core/modules/rest/tests/src/Unit/UserLoginResourceTest.php
index 3bd3786..fdd4113 100644
--- a/core/modules/rest/tests/src/Unit/UserLoginResourceTest.php
+++ b/core/modules/rest/tests/src/Unit/UserLoginResourceTest.php
@@ -83,6 +83,14 @@ public function getProtectedMethod($method) {
 
   /**
    * @expectedException \Symfony\Component\HttpKernel\Exception\BadRequestHttpException
+   * @expectedExceptionMessage No op found. Use: status, login, logout.
+   */
+  public function testEmptyPayload() {
+    $this->testClass->post([]);
+  }
+
+  /**
+   * @expectedException \Symfony\Component\HttpKernel\Exception\BadRequestHttpException
    * @expectedExceptionMessage Missing credentials.
    */
   public function testMissingCredentials() {
@@ -91,7 +99,7 @@ public function testMissingCredentials() {
 
   /**
    * @expectedException \Symfony\Component\HttpKernel\Exception\BadRequestHttpException
-   * @expectedExceptionMessage Unsupported op.
+   * @expectedExceptionMessage Unsupported op UnsuportedOp.
    */
   public function testUnsupportedOp() {
     $this->testClass->post(['op'=>'UnsuportedOp']);
@@ -99,7 +107,7 @@ public function testUnsupportedOp() {
 
   /**
    * @expectedException \Symfony\Component\HttpKernel\Exception\BadRequestHttpException
-   * @expectedExceptionMessage Missing credentials.name.
+   * @expectedExceptionMessage Missing credentials.
    */
   public function testLoginMissingCredentialName() {
     $method = $this->getProtectedMethod('login');