diff --git a/core/lib/Drupal/Core/Datetime/DateFormatter.php b/core/lib/Drupal/Core/Datetime/DateFormatter.php
index d784743cd54..e7ded8c538a 100644
--- a/core/lib/Drupal/Core/Datetime/DateFormatter.php
+++ b/core/lib/Drupal/Core/Datetime/DateFormatter.php
@@ -146,6 +146,9 @@ public function format($timestamp, $type = 'medium', $format = '', $timezone = N
    * {@inheritdoc}
    */
   public function formatInterval($interval, $granularity = 2, $langcode = NULL) {
+    if (!is_int($interval)) {
+      throw new \RuntimeException('$interval is not a string. It is a: ' . gettype($interval));
+    }
     $output = '';
     foreach ($this->units as $key => $value) {
       $key = explode('|', $key);