sveltejs/kit · error · Error

The ${port_header} header specified ${port} which is an inva

Error message

The ${port_header} header specified ${port} which is an invalid port because it is not a number. The value should only contain the port number (e.g. 443)

What it means

If PORT_HEADER is configured, its value must be a pure numeric port (e.g. 443), because it is appended to `protocol://host` to rebuild the origin. A non-numeric value like 'https,443' or '443/tcp' makes isNaN(+port) true and the adapter throws.

Source

Thrown at packages/adapter-bun/src/handler.js:88

		(protocol_header && request.headers.get(protocol_header)) || 'https'
	);
	if (!/^https?$/i.test(protocol)) {
		throw new Error(
			`The ${protocol_header} header specified ${protocol} which is an invalid protocol scheme. It should only contain the protocol scheme (e.g. \`https\`)`
		);
	}

	const host =
		(host_header && request.headers.get(host_header)) || (request.headers.get('host') ?? url.host);
	if (!host) {
		throw new Error(
			`Could not determine host from the ${host_header ? `${host_header} or ` : ''}host header`
		);
	}

	const port = port_header ? request.headers.get(port_header) : null;
	if (port && isNaN(+port)) {
		throw new Error(
			`The ${port_header} header specified ${port} which is an invalid port because it is not a number. The value should only contain the port number (e.g. 443)`
		);
	}

	// canonicalized so the caller's comparison with url.origin matches (case, default ports)
	return new URL(`${protocol}://${host}${port ? `:${port}` : ''}`).origin;
}

/**
 * @param {Request} request
 * @param {BunServer<undefined>} bun_server
 * @returns {string}
 */
function get_client_address(request, bun_server) {
	if (!address_header) {
		// requestIP() is null over unix sockets; undefined matches adapter-node
		return /** @type {string} */ (bun_server.requestIP(request)?.address);
	}

View on GitHub (pinned to 03f1687fe6)

Solutions

Configure the outermost/innermost proxy to send a single numeric port, e.g. nginx: `proxy_set_header X-Forwarded-Port $server_port;`.
Strip the header at the trusted edge so only one value survives: `proxy_set_header X-Forwarded-Port $http_x_forwarded_port;` replacing multi-value input.
Remove the PORT_HEADER config if you don't need a custom port header.

Example fix

// before
proxy_set_header X-Forwarded-Port "$http_host";
// after
proxy_set_header X-Forwarded-Port $server_port;

Defensive patterns

Strategy: validation

Validate before calling

const port = req.headers.get('x-forwarded-port');
if (port && isNaN(Number(port.trim()))) throw new Error(`Non-numeric forwarded port: ${port}`);

Try / catch

try {
  origin = getOrigin(req);
} catch (err) {
  if (/is an invalid port/.test(err.message)) {
    console.error('Proxy must send a single numeric port in the port header');
  }
  throw err;
}

Prevention

Use $server_port (nginx) or equivalent for the forwarded port header.
Overwrite rather than append multi-hop port values at the trusted edge.
Don't set PORT_HEADER unless you need origin-port fidelity.

When it happens

Trigger: A proxy sends a composite value in the port header such as `x-forwarded-port: 8443,8443` (multiple hops) or `host:443` including the host part.

Common situations: Multi-layer proxies appending to X-Forwarded-Port; misconfigured proxy forwarding the full host:port pair into the port header.

Related errors

AI-assisted analysis of sveltejs/kit@03f1687fe6 (2026-09-02). Data as JSON: /api/errors/1d789f783287f542. Report an issue: GitHub.