diff --git a/core/lib/Drupal/Core/Datetime/DateFormatter.php b/core/lib/Drupal/Core/Datetime/DateFormatter.php
index eb107e8e3931526d5045303dfb39c20477eb6fb3..421b03b51eb31c09e9eadbe8aee1d27f332879d8 100644
--- a/core/lib/Drupal/Core/Datetime/DateFormatter.php
+++ b/core/lib/Drupal/Core/Datetime/DateFormatter.php
@@ -122,11 +122,15 @@ public function format($timestamp, $type = 'medium', $format = '', $timezone = N
     $date = DrupalDateTime::createFromTimestamp($timestamp, $this->timezones[$timezone], $create_settings);
 
     // If we have a non-custom date format use the provided date format pattern.
-    if ($date_format = $this->dateFormat($type, $langcode)) {
-      $format = $date_format->getPattern();
+    if ($type !== 'custom') {
+      if ($date_format = $this->dateFormat($type, $langcode)) {
+        $format = $date_format->getPattern();
+      }
     }
 
-    // Fall back to medium if a format was not found.
+    // Fall back to the 'medium' date format type if the format string is
+    // empty, either from not finding a requested date format or being given an
+    // empty custom format string.
     if (empty($format)) {
       $format = $this->dateFormat('fallback', $langcode)->getPattern();
     }
@@ -316,9 +320,9 @@ public function formatDiff($from, $to, $options = array()) {
    * @param string $langcode
    *   The langcode of the language to use.
    *
-   * @return string|null
-   *   The pattern for the date format in the given language for non-custom
-   *   formats, NULL otherwise.
+   * @return \Drupal\Core\Datetime\DateFormatInterface|null
+   *   The configuration entity for the date format in the given language for
+   *   non-custom formats, NULL otherwise.
    */
   protected function dateFormat($format, $langcode) {
     if (!isset($this->dateFormats[$format][$langcode])) {
diff --git a/core/tests/Drupal/KernelTests/Core/Datetime/FormatDateTest.php b/core/tests/Drupal/KernelTests/Core/Datetime/FormatDateTest.php
index 5fbb4dff088f0ae0a7ca8e13171e7812eb1f7094..036b385ae43abd3534d5c97d5a77528842db71e6 100644
--- a/core/tests/Drupal/KernelTests/Core/Datetime/FormatDateTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Datetime/FormatDateTest.php
@@ -51,6 +51,8 @@ protected function setUp() {
   public function testFormatDate() {
     /** @var \Drupal\Core\Datetime\DateFormatterInterface $formatter */
     $formatter = $this->container->get('date.formatter');
+    /** @var \Drupal\Core\Language\LanguageManagerInterface $language_manager */
+    $language_manager = $this->container->get('language_manager');
 
     $timestamp = strtotime('2007-03-26T00:00:00+00:00');
     $this->assertSame('Sunday, 25-Mar-07 17:00:00 PDT', $formatter->format($timestamp, 'custom', 'l, d-M-y H:i:s T', 'America/Los_Angeles', 'en'), 'Test all parameters.');
@@ -59,6 +61,14 @@ public function testFormatDate() {
     $this->assertSame('\\domingo, 25-Mar-07 17:00:00 PDT', $formatter->format($timestamp, 'custom', '\\\\l, d-M-y H:i:s T', 'America/Los_Angeles', self::LANGCODE), 'Test format containing backslash character.');
     $this->assertSame('\\l, 25-Mar-07 17:00:00 PDT', $formatter->format($timestamp, 'custom', '\\\\\\l, d-M-y H:i:s T', 'America/Los_Angeles', self::LANGCODE), 'Test format containing backslash followed by escaped format string.');
     $this->assertSame('Monday, 26-Mar-07 01:00:00 BST', $formatter->format($timestamp, 'custom', 'l, d-M-y H:i:s T', 'Europe/London', 'en'), 'Test a different time zone.');
+    $this->assertSame('Thu, 01/01/1970 - 00:00', $formatter->format(0, 'custom', '', 'UTC', 'en'), 'Test custom format with empty string.');
+
+    // Make sure we didn't change the configuration override language.
+    $this->assertSame('en', $language_manager->getConfigOverrideLanguage()->getId(), 'Configuration override language not disturbed,');
+
+    // Test bad format string will use the fallback format.
+    $this->assertSame($formatter->format($timestamp, 'fallback'), $formatter->format($timestamp, 'bad_format_string'), 'Test fallback format.');
+    $this->assertSame('en', $language_manager->getConfigOverrideLanguage()->getId(), 'Configuration override language not disturbed,');
 
     // Change the default language and timezone.
     $this->config('system.site')->set('default_langcode', static::LANGCODE)->save();
@@ -66,7 +76,8 @@ public function testFormatDate() {
 
     // Reset the language manager so new negotiations attempts will fall back on
     // on the new language.
-    $this->container->get('language_manager')->reset();
+    $language_manager->reset();
+    $this->assertSame('en', $language_manager->getConfigOverrideLanguage()->getId(), 'Configuration override language not disturbed,');
 
     $this->assertSame('Sunday, 25-Mar-07 17:00:00 PDT', $formatter->format($timestamp, 'custom', 'l, d-M-y H:i:s T', 'America/Los_Angeles', 'en'), 'Test a different language.');
     $this->assertSame('Monday, 26-Mar-07 01:00:00 BST', $formatter->format($timestamp, 'custom', 'l, d-M-y H:i:s T', 'Europe/London'), 'Test a different time zone.');
@@ -84,6 +95,13 @@ public function testFormatDate() {
     $this->assertSame('2007-03', $formatter->format($timestamp, 'html_month'), 'Test html_month date format.');
     $this->assertSame('2007', $formatter->format($timestamp, 'html_year'), 'Test html_year date format.');
 
+    // Make sure we didn't change the configuration override language.
+    $this->assertSame('en', $language_manager->getConfigOverrideLanguage()->getId(), 'Configuration override language not disturbed,');
+
+    // Test bad format string will use the fallback format.
+    $this->assertSame($formatter->format($timestamp, 'fallback'), $formatter->format($timestamp, 'bad_format_string'), 'Test fallback format.');
+    $this->assertSame('en', $language_manager->getConfigOverrideLanguage()->getId(), 'Configuration override language not disturbed,');
+
     // HTML is not escaped by the date formatter, it must be escaped later.
     $this->assertSame("<script>alert('2007');</script>", $formatter->format($timestamp, 'custom', '\<\s\c\r\i\p\t\>\a\l\e\r\t\(\'Y\'\)\;\<\/\s\c\r\i\p\t\>'), 'Script tags not removed from dates.');
     $this->assertSame('<em>2007</em>', $formatter->format($timestamp, 'custom', '\<\e\m\>Y\<\/\e\m\>'), 'Em tags are not removed from dates.');