Start + https preview server fails on res.setHeaders is not a function

Which project does this relate to?

Start

Describe the bug

When running the preview server in SPA mode with HTTPS enabled, the server fails to serve the application shell. Instead, it crashes with an error below:

TypeError: res.setHeaders is not a function
    at file:///X:/experiments/tanstack-preview-server-https-bug/node_modules/@tanstack/start-plugin-core/dist/esm/preview-server-plugin/plugin.js:31:19

Development server with HTTPS certificates works fine.

Your Example Website or App

https://github.com/susickypavel/tanstack-preview-server-https-bug

Steps to Reproduce the Bug or Issue

1. Clone the repository

git clone git@github.com:susickypavel/tanstack-preview-server-https-bug.git
cd ./tanstack-preview-server-https-bug

2. Create certificates for example.localhost

mkdir .certs
cd .certs
mkcert example.localhost "*.example.localhost"

3. Add example.localhost to hosts file

# For Mac or Linux
nano /etc/hosts

127.0.0.1 example.localhost

4. Install dependencies and run the preview server

bun install
bun run build:preview

Expected behavior

I'd expect the preview server to work the same way it works for bun dev, but it should use the production files instead.

Screenshots or Videos

No response

Platform

• Router / Start Version: 1.132.0
• OS: Windows
• Browser: All
• Bundler: vite

Additional context

No response

Clone the linked reproduction, install its dependencies, and run bun run build:preview after creating the HTTPS certificates. Start with the stack-trace location in @tanstack/start-plugin-core/dist/esm/preview-server-plugin/plugin.js, then trace the corresponding preview-server code. Done means the HTTPS SPA preview serves the application shell using the production files without the res.setHeaders error.