diff --git a/src/Plugin/Notifier/DigestBase.php b/src/Plugin/Notifier/DigestBase.php
index e098255..753ad52 100644
--- a/src/Plugin/Notifier/DigestBase.php
+++ b/src/Plugin/Notifier/DigestBase.php
@@ -136,7 +136,7 @@ abstract class DigestBase extends MessageNotifierBase implements ContainerFactor
       $mid = $message->original_message->id();
     }
 
-    assert('!empty($mid)', 'The message entity (or $message->original_message) must be saved in order to create a digest entry.');
+    assert(!empty($mid), 'The message entity (or $message->original_message) must be saved in order to create a digest entry.');
     $message_digest['mid'] = $mid;
 
     $this->connection->insert('message_digest')->fields($message_digest)->execute();