diff --git a/src/email/postmark.ts b/src/email/postmark.ts
index 620fe9f..87bb496 100644
--- a/src/email/postmark.ts
+++ b/src/email/postmark.ts
@@ -77,7 +77,7 @@ export async function processPostmarkInboundEmail(req: Request, res: Response) {
       from: myToEmails[0] ? `"${project?.event?.name || 'CodeDay'}" <${myToEmails[0]}>` : config.email.from,
       to: email.FromFull.Email,
       subject: `Re: ${email.Subject}`,
-      html: `<p>No one received your message. It was only sent to this unmonitored mailbox. <strong>You likely forgot to reply-all.</strong></p>\n${project?.event?.emailSignature}\n<br /><br /><blockquote>${email.HtmlBody}</blockquote>`
+      html: `<p>No one received your message. It was only sent to this unmonitored mailbox. <strong>You likely forgot to reply-all.</strong></p>\n${project?.event?.emailSignature || ''}\n<br /><br /><blockquote>${email.HtmlBody}</blockquote>`
     });
     return res.send('ok');
   }
diff --git a/tests/testPostmarkReplyOneLeak.ts b/tests/testPostmarkReplyOneLeak.ts
new file mode 100644
index 0000000..e3005a3
--- /dev/null
+++ b/tests/testPostmarkReplyOneLeak.ts
@@ -0,0 +1,75 @@
+import 'reflect-metadata';
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import Container from 'typedi';
+import { PrismaClient } from '@prisma/client';
+import { processPostmarkInboundEmail } from '../src/email/postmark';
+
+const INBOUND_DOMAIN = 'labs.example';
+const TRACKING_ADDR = `cid12345+es_abc123@${INBOUND_DOMAIN}`;
+
+function mockRes() {
+  return {
+    send: () => 'ok',
+    status: () => ({ send: () => 'ok' }),
+  } as any;
+}
+
+function makeReq() {
+  return {
+    body: {
+      FromFull: { Email: 'sender@example.com' },
+      Subject: 'Hello',
+      HtmlBody: 'ORIGINAL-BODY',
+      ToFull: [{ Email: TRACKING_ADDR }],
+      CcFull: [],
+      BccFull: [],
+    },
+  } as any;
+}
+
+function installMocks(projectResult: any) {
+  const captured: { html?: string } = {};
+  Container.set(PrismaClient as any, {
+    project: { findUnique: async () => projectResult },
+  });
+  Container.set('email', {
+    sendMail: async (args: any) => {
+      captured.html = args.html;
+      return { MessageID: 'mid' };
+    },
+  });
+  return captured;
+}
+
+// Guards the reply-one auto-reply body against leaking literal "null"/"undefined"
+// when project/event/emailSignature is missing. The `from` field already falls
+// back via `|| 'CodeDay'`; this lock the same treatment for the signature in `html`.
+test('reply-one to an event with emailSignature = null does not leak literal "null"', async () => {
+  const captured = await installMocks({ event: { name: 'CodeDay Lab', emailSignature: null } });
+  await processPostmarkInboundEmail(makeReq(), mockRes());
+
+  assert.equal(typeof captured.html, 'string');
+  assert.ok(!captured.html!.includes('\nnull\n'), 'html must not contain literal "\\nnull\\n"');
+  assert.ok(!captured.html!.includes('\nundefined\n'), 'html must not contain literal "\\nundefined\\n"');
+});
+
+test('reply-one to a deleted project does not leak literal "undefined" and falls back the sender name', async () => {
+  const captured = await installMocks(null);
+  await processPostmarkInboundEmail(makeReq(), mockRes());
+
+  assert.equal(typeof captured.html, 'string');
+  assert.ok(!captured.html!.includes('\nnull\n'), 'html must not contain literal "\\nnull\\n"');
+  assert.ok(!captured.html!.includes('\nundefined\n'), 'html must not contain literal "\\nundefined\\n"');
+});
+
+test('a real emailSignature is still rendered verbatim', async () => {
+  const signature = '-- The CodeDay Lab Team';
+  const captured = await installMocks({ event: { name: 'CodeDay Lab', emailSignature: signature } });
+  await processPostmarkInboundEmail(makeReq(), mockRes());
+
+  assert.equal(typeof captured.html, 'string');
+  assert.ok(captured.html!.includes(`\n${signature}\n`), 'signature must be interpolated verbatim');
+  assert.ok(!captured.html!.includes('\nnull\n'));
+  assert.ok(!captured.html!.includes('\nundefined\n'));
+});