diff --git a/src/infrastructure/setup/PhabricatorSetup.php b/src/infrastructure/setup/PhabricatorSetup.php
index a740cce548..d65f4c3c11 100644
--- a/src/infrastructure/setup/PhabricatorSetup.php
+++ b/src/infrastructure/setup/PhabricatorSetup.php
@@ -735,12 +735,19 @@ final class PhabricatorSetup {
     }
 
     self::writeHeader('CONFIG CLASSES');
-    foreach (PhabricatorEnv::getRequiredClasses() as $key => $class) {
-      if (!PhabricatorEnv::getEnvConfig($key)) {
+    foreach (PhabricatorEnv::getRequiredClasses() as $key => $instanceof) {
+      $config = PhabricatorEnv::getEnvConfig($key);
+      if (!$config) {
         self::writeNote("'$key' is not set.");
       } else {
         try {
-          PhabricatorEnv::newObjectFromConfig($key);
+          $r = new ReflectionClass($config);
+          if (!$r->isSubclassOf($instanceof)) {
+            throw new Exception(
+              "Config setting '$key' must be an instance of '$instanceof'.");
+          } elseif (!$r->isInstantiable()) {
+            throw new Exception("Config setting '$key' must be instantiable.");
+          }
         } catch (Exception $ex) {
           self::writeFailure();
           self::write("Setup failure! ".$ex->getMessage());