diff --git a/framework/core/src/Extend/Conditional.php b/framework/core/src/Extend/Conditional.php
index 55584beeb..f9098430a 100644
--- a/framework/core/src/Extend/Conditional.php
+++ b/framework/core/src/Extend/Conditional.php
@@ -82,12 +82,15 @@ class Conditional implements ExtenderInterface
             if ($condition['condition']) {
                 $extenders = $condition['extenders'];
 
-                if (is_string($extenders)) {
-                    $extenders = $container->call($extenders);
+                if (is_string($extenders) && class_exists($extenders) && method_exists($extenders, '__invoke')) {
+                    $result = $container->call($extenders);
+                    $extenders = is_array($result) ? $result : [$result];
                 } elseif (is_callable($extenders)) {
                     $extenders = $container->call($extenders);
                 }
 
+                assert(is_array($extenders), 'Extenders should be an array after resolution.');
+
                 foreach ($extenders as $extender) {
                     $extender->extend($container, $extension);
                 }