diff --git a/src/applications/differential/mail/base/DifferentialMail.php b/src/applications/differential/mail/base/DifferentialMail.php
index 6e506d6486..67b1b963a7 100644
--- a/src/applications/differential/mail/base/DifferentialMail.php
+++ b/src/applications/differential/mail/base/DifferentialMail.php
@@ -215,27 +215,19 @@ EOTEXT;
   }
 
   public function getReplyHandler() {
-    if ($this->replyHandler) {
-      return $this->replyHandler;
+    if (!$this->replyHandler) {
+      $this->replyHandler =
+        self::newReplyHandlerForRevision($this->getRevision());
     }
 
-    $handler_class = PhabricatorEnv::getEnvConfig(
-      'metamta.differential.reply-handler');
-
-    $reply_handler = self::newReplyHandlerForRevision($this->getRevision());
-
-    $this->replyHandler = $reply_handler;
-
     return $this->replyHandler;
   }
 
   public static function newReplyHandlerForRevision(
     DifferentialRevision $revision) {
 
-    $handler_class = PhabricatorEnv::getEnvConfig(
+    $reply_handler = PhabricatorEnv::newObjectFromConfig(
       'metamta.differential.reply-handler');
-
-    $reply_handler = newv($handler_class, array());
     $reply_handler->setMailReceiver($revision);
 
     return $reply_handler;
diff --git a/src/infrastructure/env/PhabricatorEnv.php b/src/infrastructure/env/PhabricatorEnv.php
index eb8f22faa0..2e3b6151c8 100644
--- a/src/infrastructure/env/PhabricatorEnv.php
+++ b/src/infrastructure/env/PhabricatorEnv.php
@@ -22,6 +22,10 @@
 final class PhabricatorEnv {
   private static $env;
 
+  private static $requiredClasses = array(
+    'metamta.differential.reply-handler' => 'PhabricatorMailReplyHandler',
+  );
+
   public static function setEnvConfig(array $config) {
     self::$env = $config;
   }
@@ -30,6 +34,17 @@ final class PhabricatorEnv {
     return idx(self::$env, $key, $default);
   }
 
+  public static function newObjectFromConfig($key, $args = array()) {
+    $class = self::getEnvConfig($key);
+    $object = newv($class, $args);
+    $instanceof = idx(self::$requiredClasses, $key);
+    if (!($object instanceof $instanceof)) {
+      throw new Exception("Config setting '$key' must be an instance of ".
+        "'$instanceof', is '".get_class($object)."'.");
+    }
+    return $object;
+  }
+
   public static function envConfigExists($key) {
     return array_key_exists($key, self::$env);
   }