diff --git a/src/Behat/MinkExtension/ServiceContainer/MinkExtension.php b/src/Behat/MinkExtension/ServiceContainer/MinkExtension.php
index a1209f8..3695371 100644
--- a/src/Behat/MinkExtension/ServiceContainer/MinkExtension.php
+++ b/src/Behat/MinkExtension/ServiceContainer/MinkExtension.php
@@ -117,7 +117,7 @@ public function configure(ArrayNodeDefinition $builder)
                 ->always()
                 ->then(function ($v) use ($renamedKeys) {
                     foreach ($renamedKeys as $driverType) {
-                        if (!array_key_exists($driverType, $v) || isset($v['sessions'][$driverType])) {
+                        if (!array_key_exists($driverType, (array)$v) || isset($v['sessions'][$driverType])) {
                             continue;
                         }