diff --git a/core/tests/Drupal/Tests/BrowserTestBase.php b/core/tests/Drupal/Tests/BrowserTestBase.php
index bd50c23b486..91f09080f41 100644
--- a/core/tests/Drupal/Tests/BrowserTestBase.php
+++ b/core/tests/Drupal/Tests/BrowserTestBase.php
@@ -320,7 +320,7 @@ protected function initMink() {
   protected function getDefaultDriverInstance() {
     // Get default driver params from environment if availables.
     if ($arg_json = getenv('MINK_DRIVER_ARGS')) {
-      $this->minkDefaultDriverArgs = json_decode($arg_json);
+      $this->minkDefaultDriverArgs = json_decode($arg_json, TRUE);
     }
 
     // Get and check default driver class from environment if availables.