diff --git a/core/lib/Drupal/Core/Installer/CheckInstalledTrait.php b/core/lib/Drupal/Core/Installer/CheckInstalledTrait.php
index 2894a76..821352a 100644
--- a/core/lib/Drupal/Core/Installer/CheckInstalledTrait.php
+++ b/core/lib/Drupal/Core/Installer/CheckInstalledTrait.php
@@ -14,6 +14,15 @@
 trait CheckInstalledTrait {

   /**
+   * Returns whether the current PHP process runs on CLI.
+   *
+   * @return bool
+   */
+  protected function isCli() {
+    return PHP_SAPI === 'cli';
+  }
+
+  /**
    * Determines if an exception handler should redirect to the installer.
    *
    * @param \Exception $exception
@@ -29,27 +38,28 @@
    *   Drupal is not installed yet, or FALSE otherwise.
    */
   protected function shouldRedirectToInstaller(\Exception $exception, Connection $connection = NULL) {
-    // To avoid unnecessary queries, only act if the exception is one that is
-    // expected to occur when Drupal has not yet been installed. This includes
-    // NotFoundHttpException because an uninstalled site won't have route
-    // information available yet and therefore can return 404 errors.
-    if (!($exception instanceof \PDOException || $exception instanceof DatabaseException || $exception instanceof NotFoundHttpException)) {
+    // Never redirect on the command line.
+    if ($this->isCli()) {
       return FALSE;
     }

-    // Never redirect on the command line.
-    if (PHP_SAPI === 'cli') {
+    // Never redirect if we're already in the installer.
+    if (drupal_installation_attempted()) {
       return FALSE;
     }

     // If the database wasn't found, assume the user hasn't entered it properly
-    // and redirect to the installer.
+    // and redirect to the installer. This check needs to come first because a
+    // DatabaseNotFoundException is also an instance of DatabaseException.
     if ($exception instanceof DatabaseNotFoundException) {
       return TRUE;
     }

-    // Never redirect if we're already in the installer.
-    if (drupal_installation_attempted()) {
+    // To avoid unnecessary queries, only act if the exception is one that is
+    // expected to occur when Drupal has not yet been installed. This includes
+    // NotFoundHttpException because an uninstalled site won't have route
+    // information available yet and therefore can return 404 errors.
+    if (!($exception instanceof \PDOException || $exception instanceof DatabaseException || $exception instanceof NotFoundHttpException)) {
       return FALSE;
     }

diff --git a/core/tests/Drupal/Tests/Core/Installer/CheckInstalledTraitTest.php b/core/tests/Drupal/Tests/Core/Installer/CheckInstalledTraitTest.php
new file mode 100644
index 0000000..abb7f20
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Installer/CheckInstalledTraitTest.php
@@ -0,0 +1,128 @@
+<?php
+
+namespace Drupal\Tests\Core\Installer;
+
+use Drupal\Core\Database\DatabaseException;
+use Drupal\Core\Database\DatabaseNotFoundException;
+use Drupal\Tests\UnitTestCase;
+use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
+
+// The function drupal_installation_attempted() has to be included.
+$root = dirname(dirname(dirname(dirname(dirname(dirname(__DIR__))))));
+require_once $root . '/core/includes/bootstrap.inc';
+
+/**
+ * @coversDefaultClass \Drupal\Core\Installer\CheckInstalledTrait
+ *
+ * @group Installer
+ */
+class CheckInstalledTraitTest extends UnitTestCase {
+
+  /**
+   * Data provider for testShouldRedirectToInstaller().
+   *
+   * @return array
+   *   - Expected result from shouldRedirectToInstaller().
+   *   - Exceptions to be handled by shouldRedirectToInstaller()
+   *   - Whether or not there is a database connection.
+   *   - Whether or not there is database connection info.
+   *   - Whether or not there exists a sessions table in the database.
+   */
+  public function providerShouldRedirectToInstaller() {
+    return array(
+      [TRUE, '\Drupal\Core\Database\DatabaseNotFoundException', FALSE, FALSE],
+      [TRUE, '\Drupal\Core\Database\DatabaseNotFoundException', TRUE, FALSE],
+      [TRUE, '\Drupal\Core\Database\DatabaseNotFoundException', FALSE, TRUE],
+      [TRUE, '\Drupal\Core\Database\DatabaseNotFoundException', TRUE, TRUE],
+      [TRUE, '\Drupal\Core\Database\DatabaseNotFoundException', TRUE, TRUE, FALSE],
+
+      [TRUE, '\PDOException', FALSE, FALSE],
+      [TRUE, '\PDOException', TRUE, FALSE],
+      [FALSE, '\PDOException', FALSE, TRUE],
+      [FALSE, '\PDOException', TRUE, TRUE],
+      [TRUE, '\PDOException', TRUE, TRUE, FALSE],
+
+      [TRUE, '\Drupal\Core\Database\DatabaseExceptionWrapper', FALSE, FALSE],
+      [TRUE, '\Drupal\Core\Database\DatabaseExceptionWrapper', TRUE, FALSE],
+      [FALSE, '\Drupal\Core\Database\DatabaseExceptionWrapper', FALSE, TRUE],
+      [FALSE, '\Drupal\Core\Database\DatabaseExceptionWrapper', TRUE, TRUE],
+      [TRUE, '\Drupal\Core\Database\DatabaseExceptionWrapper', TRUE, TRUE, FALSE],
+
+      [TRUE, '\Symfony\Component\HttpKernel\Exception\NotFoundHttpException', FALSE, FALSE],
+      [TRUE, '\Symfony\Component\HttpKernel\Exception\NotFoundHttpException', TRUE, FALSE],
+      [FALSE, '\Symfony\Component\HttpKernel\Exception\NotFoundHttpException', FALSE, TRUE],
+      [FALSE, '\Symfony\Component\HttpKernel\Exception\NotFoundHttpException', TRUE, TRUE],
+      [TRUE, '\Symfony\Component\HttpKernel\Exception\NotFoundHttpException', TRUE, TRUE, FALSE],
+
+      [FALSE, '\Exception', FALSE, FALSE],
+      [FALSE, '\Exception', TRUE, FALSE],
+      [FALSE, '\Exception', FALSE, TRUE],
+      [FALSE, '\Exception', TRUE, TRUE],
+      [FALSE, '\Exception', TRUE, TRUE, FALSE],
+    );
+  }
+
+  /**
+   * @covers ::shouldRedirectToInstaller
+   * @dataProvider providerShouldRedirectToInstaller
+   */
+  function testShouldRedirectToInstaller($expected, $exception, $connection, $connection_info, $session_table_exists = TRUE) {
+    try {
+      throw new $exception();
+    }
+    catch (\Exception $e) {
+      // Mock the trait.
+      $trait = $this->getMockBuilder('Drupal\Core\Installer\CheckInstalledTrait')
+        ->setMethods(array('isCli'))
+        ->getMockForTrait();
+
+      // Make sure that the method thinks we are not using the cli.
+      $trait->expects($this->any())
+        ->method('isCli')
+        ->willReturn(FALSE);
+
+      // Un-protect the method using reflection.
+      $method_ref = new \ReflectionMethod($trait, 'shouldRedirectToInstaller');
+      $method_ref->setAccessible(TRUE);
+
+      // Mock the database connection info.
+      $db = $this->getMockForAbstractClass('Drupal\Core\Database\Database');
+      $property_ref = new \ReflectionProperty($db, 'databaseInfo');
+      $property_ref->setAccessible(true);
+      $property_ref->setValue($db, ['default' => $connection_info]);
+
+      if ($connection) {
+        // Mock the database connection.
+        $connection = $this->getMockBuilder('Drupal\Core\Database\Connection')
+          ->disableOriginalConstructor()
+          ->setMethods(array('schema'))
+          ->getMockForAbstractClass();
+
+        if ($connection_info) {
+          // Mock the database schema class.
+          $schema = $this->getMockBuilder('Drupal\Core\Database\Schema')
+            ->disableOriginalConstructor()
+            ->setMethods(array('tableExists'))
+            ->getMockForAbstractClass();
+
+          $schema->expects($this->any())
+            ->method('tableExists')
+            ->with('sessions')
+            ->willReturn($session_table_exists);
+
+          $connection->expects($this->any())
+            ->method('schema')
+            ->willReturn($schema);
+        }
+      }
+      else {
+        // Set the database connection if there is none.
+        $connection = NULL;
+      }
+
+      // Call shouldRedirectToInstaller.
+      $this->assertSame($expected, $method_ref->invoke($trait, $e, $connection));
+    }
+  }
+
+}