diff --git a/CRM/Mosaico/Services.php b/CRM/Mosaico/Services.php
index 44e7847f7e..ef4e910bd1 100644
--- a/CRM/Mosaico/Services.php
+++ b/CRM/Mosaico/Services.php
@@ -32,7 +32,10 @@ public static function registerServices(ContainerBuilder $container) {
   protected static function getListenerSpecs() {
     $listenerSpecs = array();
 
-    $listenerSpecs[] = array(\Civi\FlexMailer\Validator::EVENT_CHECK_SENDABLE, array('mosaico_required_tokens', 'onCheckSendable'), FM::WEIGHT_MAIN);
+    if (class_exists('\Civi\FlexMailer\Validator')) {
+      // TODO Simplify by removing conditional. Wait until at least Feb 2018.
+      $listenerSpecs[] = array(\Civi\FlexMailer\Validator::EVENT_CHECK_SENDABLE, array('mosaico_required_tokens', 'onCheckSendable'), FM::WEIGHT_MAIN);
+    }
     $listenerSpecs[] = array(FM::EVENT_COMPOSE, array('mosaico_flexmail_composer', 'onCompose'), FM::WEIGHT_MAIN);
     $listenerSpecs[] = array(FM::EVENT_COMPOSE, array('mosaico_flexmail_url_filter', 'onCompose'), FM::WEIGHT_ALTER - 100);