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);
diff --git a/core/scripts/run-tests.sh b/core/scripts/run-tests.sh
index 40ed2b3f2f4..2163902ba14 100755
--- a/core/scripts/run-tests.sh
+++ b/core/scripts/run-tests.sh
@@ -1311,7 +1311,7 @@ function simpletest_script_reporter_write_xml_results() {
 function simpletest_script_reporter_timer_stop() {
   echo "\n";
   $end = Timer::stop('run-tests');
-  echo "Test run duration: " . \Drupal::service('date.formatter')->formatInterval($end['time'] / 1000);
+  echo "Test run duration: " . \Drupal::service('date.formatter')->formatInterval((int) ($end['time'] / 1000));
   echo "\n\n";
 }