diff --git a/core/lib/Drupal/Core/StringTranslation/TranslatableMarkup.php b/core/lib/Drupal/Core/StringTranslation/TranslatableMarkup.php
index 8b4902a0ad75a2465a857b88b19c618b31dab4be..82b76a0da84154470d0a9b03bd869b55105d36c8 100644
--- a/core/lib/Drupal/Core/StringTranslation/TranslatableMarkup.php
+++ b/core/lib/Drupal/Core/StringTranslation/TranslatableMarkup.php
@@ -72,9 +72,16 @@ class TranslatableMarkup extends FormattableMarkup {
    * @param \Drupal\Core\StringTranslation\TranslationInterface $string_translation
    *   (optional) The string translation service.
    *
+   * @throws \InvalidArgumentException
+   *   Exception thrown when $string is not a string.
+   *
    * @see \Drupal\Component\Render\FormattableMarkup::placeholderFormat()
    */
   public function __construct($string, array $arguments = array(), array $options = array(), TranslationInterface $string_translation = NULL) {
+    if (!is_string($string)) {
+      $message = $string instanceof TranslatableMarkup ? '$string ("' . $string->getUntranslatedString() . '") must be a string.' : '$string ("' . (string) $string . '") must be a string.';
+      throw new \InvalidArgumentException($message);
+    }
     $this->string = $string;
     $this->arguments = $arguments;
     $this->options = $options;
diff --git a/core/lib/Drupal/Core/Validation/DrupalTranslator.php b/core/lib/Drupal/Core/Validation/DrupalTranslator.php
index c7db71cd34d31eec8e34019ad0e1e61130c45545..da9c11a9e4ba30db846ac8146df7ebc58f91a410 100644
--- a/core/lib/Drupal/Core/Validation/DrupalTranslator.php
+++ b/core/lib/Drupal/Core/Validation/DrupalTranslator.php
@@ -8,6 +8,7 @@
 namespace Drupal\Core\Validation;
 
 use Drupal\Component\Render\MarkupInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
 
 /**
  * Translates strings using Drupal's translation system.
@@ -27,8 +28,9 @@ class DrupalTranslator implements TranslatorInterface {
    * Implements \Symfony\Component\Translation\TranslatorInterface::trans().
    */
   public function trans($id, array $parameters = array(), $domain = NULL, $locale = NULL) {
-
-    return t($id, $this->processParameters($parameters), $this->getOptions($domain, $locale));
+    // If a TranslatableMarkup object is passed in as $id, return it since the
+    // message has already been translated.
+    return $id instanceof TranslatableMarkup ? $id : t($id, $this->processParameters($parameters), $this->getOptions($domain, $locale));
   }
 
   /**
diff --git a/core/modules/field/src/Tests/FormTest.php b/core/modules/field/src/Tests/FormTest.php
index 9e70b3821cec0b40d76a1981fd3dd17e98584c9a..ce0001171848dc6c8e33cd525f124ef624b162a1 100644
--- a/core/modules/field/src/Tests/FormTest.php
+++ b/core/modules/field/src/Tests/FormTest.php
@@ -23,9 +23,11 @@ class FormTest extends FieldTestBase {
   /**
    * Modules to enable.
    *
+   * Locale is installed so that TranslatableMarkup actually does something.
+   *
    * @var array
    */
-  public static $modules = array('node', 'field_test', 'options', 'entity_test');
+  public static $modules = array('node', 'field_test', 'options', 'entity_test', 'locale');
 
   /**
    * An array of values defining a field single.
diff --git a/core/modules/system/tests/modules/module_test/module_test.module b/core/modules/system/tests/modules/module_test/module_test.module
index cead3299013fb6d1ade04d191325cb90cfe5ab3f..ff7af5742ba0f81297b14a8eb5256b259392083c 100644
--- a/core/modules/system/tests/modules/module_test/module_test.module
+++ b/core/modules/system/tests/modules/module_test/module_test.module
@@ -47,7 +47,7 @@ function module_test_system_info_alter(&$info, Extension $file, $type) {
     }
   }
   if ($file->getName() == 'seven' && $type == 'theme') {
-    $info['regions']['test_region'] = t('Test region');
+    $info['regions']['test_region'] = 'Test region';
   }
 }
 
diff --git a/core/tests/Drupal/Tests/Core/StringTranslation/TranslatableMarkupTest.php b/core/tests/Drupal/Tests/Core/StringTranslation/TranslatableMarkupTest.php
index 0b243ccd5e4871ac89575e789c2ea6f10668e88e..b038fd0cee416af2d3850620b30e46780d10e128 100644
--- a/core/tests/Drupal/Tests/Core/StringTranslation/TranslatableMarkupTest.php
+++ b/core/tests/Drupal/Tests/Core/StringTranslation/TranslatableMarkupTest.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\Tests\Core\StringTranslation;
 
+use Drupal\Component\Render\FormattableMarkup;
 use Drupal\Core\StringTranslation\TranslationInterface;
 use Drupal\Core\StringTranslation\TranslatableMarkup;
 use Drupal\Tests\UnitTestCase;
@@ -85,4 +86,27 @@ public function testToString() {
     $this->assertRegExp('/Exception thrown while calling __toString on a .*Mock_TranslatableMarkup_.* object in .*TranslatableMarkupTest.php on line [0-9]+: Yes you may./', $this->lastErrorMessage);
   }
 
+  /**
+   * @expectedException \InvalidArgumentException
+   * @expectedExceptionMessage $string ("foo") must be a string.
+   *
+   * @covers ::__construct
+   */
+  public function testIsStringAssertion() {
+    $translation = $this->getStringTranslationStub();
+    new TranslatableMarkup(new TranslatableMarkup('foo', [], [], $translation));
+  }
+
+  /**
+   * @expectedException \InvalidArgumentException
+   * @expectedExceptionMessage $string ("foo") must be a string.
+   *
+   * @covers ::__construct
+   */
+  public function testIsStringAssertionWithFormattableMarkup() {
+    $translation = $this->getStringTranslationStub();
+    $formattable_string = new FormattableMarkup('@bar', ['@bar' => 'foo']);
+    new TranslatableMarkup($formattable_string);
+  }
+
 }