diff --git a/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/UniqueFieldConstraint.php b/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/UniqueFieldConstraint.php
index 430a612..c917cc9 100644
--- a/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/UniqueFieldConstraint.php
+++ b/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/UniqueFieldConstraint.php
@@ -19,7 +19,7 @@
  */
 class UniqueFieldConstraint extends Constraint {
 
-  public $message = 'The %entity_type with the %field_name %value already exists.';
+  public $message = 'The @entity_type with the @field_name %value already exists.';
 
   /**
    * {@inheritdoc}
diff --git a/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/UniqueFieldValueValidator.php b/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/UniqueFieldValueValidator.php
index 6fbf559..1917c5c 100644
--- a/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/UniqueFieldValueValidator.php
+++ b/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/UniqueFieldValueValidator.php
@@ -40,8 +40,8 @@ public function validate($items, Constraint $constraint) {
     if ($value_taken) {
       $this->context->addViolation($constraint->message, [
         '%value' => $item->value,
-        '%entity_type' => $entity->getEntityType()->getLowercaseLabel(),
-        '%field_name' => Unicode::strtolower($items->getFieldDefinition()->getLabel()),
+        '@entity_type' => $entity->getEntityType()->getLowercaseLabel(),
+        '@field_name' => Unicode::strtolower($items->getFieldDefinition()->getLabel()),
       ]);
     }
   }
diff --git a/core/modules/block_content/src/Entity/BlockContent.php b/core/modules/block_content/src/Entity/BlockContent.php
index 37b343d..40c19e9 100644
--- a/core/modules/block_content/src/Entity/BlockContent.php
+++ b/core/modules/block_content/src/Entity/BlockContent.php
@@ -188,7 +188,7 @@ public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
         'weight' => -5,
       ))
       ->setDisplayConfigurable('form', TRUE)
-      ->addConstraint('UniqueField', ['message' => 'A block with description %value already exists.']);
+      ->addConstraint('UniqueField');
 
     $fields['type'] = BaseFieldDefinition::create('entity_reference')
       ->setLabel(t('Block type'))
diff --git a/core/modules/block_content/src/Tests/BlockContentCreationTest.php b/core/modules/block_content/src/Tests/BlockContentCreationTest.php
index d2bef6e..66c259a 100644
--- a/core/modules/block_content/src/Tests/BlockContentCreationTest.php
+++ b/core/modules/block_content/src/Tests/BlockContentCreationTest.php
@@ -66,8 +66,8 @@ public function testBlockContentCreation() {
     $this->drupalPostForm('block/add/basic', $edit, t('Save'));
 
     // Check that the Basic block has been created.
-    $this->assertRaw(format_string('A block with description %name already exists.', array(
-      '%name' => $edit['info[0][value]']
+    $this->assertRaw(format_string('The custom block with the block description %value already exists.', array(
+      '%value' => $edit['info[0][value]']
     )));
     $this->assertResponse(200);
   }
@@ -124,7 +124,7 @@ public function testBlockContentCreationMultipleViewModes() {
     $this->drupalPostForm('block/add/basic', $edit, t('Save'));
 
     // Check that the Basic block has been created.
-    $this->assertRaw(format_string('A block with description %value already exists.', array(
+    $this->assertRaw(format_string('The custom block with the block description %value already exists.', array(
       '%value' => $edit['info[0][value]']
     )));
     $this->assertResponse(200);
diff --git a/core/modules/block_content/src/Tests/BlockContentValidationTest.php b/core/modules/block_content/src/Tests/BlockContentValidationTest.php
index 85bf6e9..e13f010 100644
--- a/core/modules/block_content/src/Tests/BlockContentValidationTest.php
+++ b/core/modules/block_content/src/Tests/BlockContentValidationTest.php
@@ -37,7 +37,7 @@ public function testValidation() {
     // Make sure the violation is on the info property
     $this->assertEqual($violations[0]->getPropertyPath(), 'info');
     // Make sure the message is correct.
-    $this->assertEqual($violations[0]->getMessage(), format_string('A block with description %value already exists.', [
+    $this->assertEqual($violations[0]->getMessage(), format_string('The custom block with the block description %value already exists.', [
       '%value' => $block->label(),
     ]));
   }
diff --git a/core/modules/user/src/Entity/User.php b/core/modules/user/src/Entity/User.php
index 0405303..2952bce 100644
--- a/core/modules/user/src/Entity/User.php
+++ b/core/modules/user/src/Entity/User.php
@@ -477,7 +477,7 @@ public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
       ->setLabel(t('Email'))
       ->setDescription(t('The email of this user.'))
       ->setDefaultValue('')
-      ->addConstraint('UniqueField', [])
+      ->addConstraint('UniqueField')
       ->addConstraint('UserMailRequired')
       ->addConstraint('ProtectedUserField');