Update to Drupal 8.2.6. For more information, see https://www.drupal.org/project/drupal/releases/8.2.6

vendor/symfony/process/ProcessUtils.php

@@ -80,7 +80,7 @@ class ProcessUtils
      * @param string $caller The name of method call that validates the input
      * @param mixed  $input  The input to validate
      *
-     * @return string The validated input
+     * @return mixed The validated input
      *
      * @throws InvalidArgumentException In case the input is not valid
      *
@@ -92,6 +92,9 @@ class ProcessUtils
             if (is_resource($input)) {
                 return $input;
             }
+            if (is_string($input)) {
+                return $input;
+            }
             if (is_scalar($input)) {
                 return (string) $input;
             }