diff --git a/backend/mailer.js b/backend/mailer.js
index d91bded..de2f35e 100644
--- a/backend/mailer.js
+++ b/backend/mailer.js
@@ -35,8 +35,9 @@ function renderTemplate(template, vars) {
   let { subject, body } = template;
   for (const [k, v] of Object.entries(vars)) {
     const re = new RegExp(`\\{\\{${k}\\}\\}`, 'g');
-    subject = subject.replace(re, v || '');
-    body = body.replace(re, v || '');
+    const val = String(v ?? '');
+    subject = subject.replace(re, () => val);
+    body = body.replace(re, () => val);
   }
   return { subject, body };
 }