diff --git a/src/Protocol/SmtpPluginManager.php b/src/Protocol/SmtpPluginManager.php
index 55839f05..9d07ceff 100644
--- a/src/Protocol/SmtpPluginManager.php
+++ b/src/Protocol/SmtpPluginManager.php
@@ -75,7 +75,7 @@ public function validate($plugin)
             throw new InvalidServiceException(sprintf(
                 'Plugin of type %s is invalid; must extend %s',
                 (is_object($plugin) ? get_class($plugin) : gettype($plugin)),
-                Smtp::class
+                $this->instanceOf
             ));
         }
     }