What causes "${keypath} cannot be empty" in sveltejs/kit?

Set the option to a non-empty string value appropriate for the option

sveltejs/kit · error · Error

${keypath} cannot be empty

Error message

${keypath} cannot be empty

What it means

SvelteKit validates certain config values (like `paths.base`) must be non-empty strings. When a value that must be a non-empty string is set to an empty string, validation throws with the config key path (keypath) in the message. It exists to catch misconfigured route/path options early before build.

Source

Thrown at packages/kit/src/core/config/options.js:81

			}

			return input;
		}),
		(keypath) =>
			`The \`${keypath}\` option is deprecated, and will be removed in a future version of SvelteKit. Use subpath imports instead: https://svelte.dev/docs/kit/$lib`
	),

	appDir: validate('_app', (input, keypath) => {
		assert_string(input, keypath);

		if (input) {
			if (input.startsWith('/') || input.endsWith('/')) {
				throw new Error(
					`${keypath} cannot start or end with '/'. See https://svelte.dev/docs/kit/configuration`
				);
			}
		} else {
			throw new Error(`${keypath} cannot be empty`);
		}

		return input;
	}),

	compilerOptions: any(),

	csp: object({
		mode: list(['auto', 'hash', 'nonce']),
		directives,
		reportOnly: directives
	}),

	csrf: object({
		checkOrigin: removed(
			(keypath) => `\`${keypath}\` has been removed in favour of \`csrf.trustedOrigins\``
		),
		trustedOrigins: string_array([])

View on GitHub (pinned to 03f1687fe6)

Solutions

Set the option to a non-empty string value appropriate for the option
Remove the key entirely so SvelteKit uses its built-in default instead of an empty string
If the value comes from an env var, provide a fallback: `process.env.MY_VAR || '/default'`

Example fix

// before
export default {
  paths: { base: '' }
};
// after
export default {
  paths: { base: '/my-app' }
};

Defensive patterns

Strategy: validation

Validate before calling

const value = config.paths?.base;
if (value !== undefined && (typeof value !== 'string' || value === '')) {
  throw new Error('paths.base must be a non-empty string (or omitted)');
}

Type guard

function isValidBase(v) {
  return v === undefined || (typeof v === 'string' && v !== '');
}

Try / catch

try {
  validateConfig(config);
} catch (e) {
  if (String(e.message).includes('cannot be empty')) {
    console.error('Fix empty config value in svelte.config.js');
  }
  throw e;
}

Prevention

Never hardcode empty strings for required config options — omit the key to use defaults
Provide fallbacks when reading env vars: `process.env.X || 'default'`
Run `vite dev` or `vite build` locally before CI to surface config validation early

When it happens

Trigger: Setting a config option validated by the empty-string guard (e.g. `paths.base: ''` is fine, but keys whose value must be non-empty) to `''` — specifically any string-validated keypath that is tested for emptiness and receives `''`.

Common situations: Copying example configs with placeholder empty values; env-driven config where an env var is unset and defaults to empty string; accidentally deleting the value in svelte.config.js.

Understand the failure class

Background: "Invalid value" and "allowed values are" config errors: what your library rejected and how to fix it — this error's family across 41 libraries.

Related errors

AI-assisted analysis of sveltejs/kit@03f1687fe6 (2026-09-02). Data as JSON: /api/errors/3068fd56d0f81e05. Report an issue: GitHub.