diff --git a/lib/webpush/request.rb b/lib/webpush/request.rb
index d90a01f..813f894 100644
--- a/lib/webpush/request.rb
+++ b/lib/webpush/request.rb
@@ -116,7 +116,7 @@ def expiration
     end
 
     def subject
-      @vapid_options.fetch(:subject, 'sender@example.com')
+      @vapid_options.fetch(:subject, 'mailto:sender@example.com')
     end
 
     def vapid_public_key