diff --git a/lib/transaction.js b/lib/transaction.js
index f7a5a04..d068106 100644
--- a/lib/transaction.js
+++ b/lib/transaction.js
@@ -51,7 +51,7 @@ class Transaction {
 
     this.body = new message.Body(this.header)
     for (const hook of this.attachment_start_hooks) {
-        this.body.on('attachment_start', h);
+        this.body.on('attachment_start', hook);
     }
 
     if (this.banner) this.body.set_banner(this.banner)