slimphp/Slim · error · InvalidArgumentException

Invalid style `{$style}`. Must be `append` or `prepend`

Error message

Invalid style `{$style}`. Must be `append` or `prepend`

What it means

Error "Invalid style `{$style}`. Must be `append` or `prepend`" thrown in slimphp/Slim.

Source

Thrown at Slim/Middleware/OutputBufferingMiddleware.php:45

class OutputBufferingMiddleware implements MiddlewareInterface
{
    public const APPEND = 'append';
    public const PREPEND = 'prepend';

    protected StreamFactoryInterface $streamFactory;

    protected string $style;

    /**
     * @param string $style Either "append" or "prepend"
     */
    public function __construct(StreamFactoryInterface $streamFactory, string $style = 'append')
    {
        $this->streamFactory = $streamFactory;
        $this->style = $style;

        if (!in_array($style, [static::APPEND, static::PREPEND], true)) {
            throw new InvalidArgumentException("Invalid style `{$style}`. Must be `append` or `prepend`");
        }
    }

    /**
     * @throws Throwable
     */
    public function process(ServerRequestInterface $request, RequestHandlerInterface $handler): ResponseInterface
    {
        try {
            ob_start();
            $response = $handler->handle($request);
            $output = ob_get_clean();
        } catch (Throwable $e) {
            ob_end_clean();
            throw $e;
        }

        if (!empty($output)) {

View on GitHub (pinned to 80900fb39c)

When it happens

Trigger: Thrown at Slim/Middleware/OutputBufferingMiddleware.php:45 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of slimphp/Slim@80900fb39c (2026-08-21). Data as JSON: /api/errors/18a39d51ce061e57. Report an issue: GitHub.