private function createResponseObject(
        RequestInterface $request,
        array $headers,
        TransactionInterface $transaction,
        StreamInterface $stream
    )    private function createResponseObject(
        RequestInterface $request,
        array $headers,
        TransactionInterface $transaction,
        StreamInterface $stream
    ) 

private function createResponse(TransactionInterface $transaction)
    {
        $request = $transaction->getRequest();
        $stream = $this->createStream($request, $http_response_header);
        $this->createResponseObject(
            $request,
            $http_response_header,
            $transaction,
            new Stream($stream)
        );
    }

 private function createStream(
        RequestInterface $request,
        &$http_response_header
    ) {
        static $methods;
        if (!$methods) {
            $methods = array_flip(get_class_methods(__CLASS__));
        }

        $params = array();
        $options = $this->getDefaultOptions($request);
        foreach ($request->getConfig()->toArray() as $key => $value) {
            $method = "add_{$key}";
            if (isset($methods[$method])) {
                $this->{$method}($request, $options, $value, $params);
            }
        }

        $this->applyCustomOptions($request, $options);
        $context = $this->createStreamContext($request, $options, $params);

        return $this->createStreamResource(
            $request,
            $options,
            $context,
            $http_response_header
        );
    }

 private function createStreamResource(
        RequestInterface $request,
        array $options,
        $context,
        &$http_response_header
    ) {
        $url = $request->getUrl();

        return $this->createResource(
            function () use ($url, &$http_response_header, $context) {
                if (false === strpos($url, 'http')) {
                    trigger_error("URL is invalid: {$url}", E_USER_WARNING);
                    return null;
                }
                return fopen($url, 'r', null, $context);
            },
            $request,
            $options
        );
    }


[code]

As result the "$http_response_header" remains [b]null[/b] , NOT [b]array[/b] and thats what triggers the TypeException is PHP.

So , of course it's possible to disable PHP type checking in PHP config, but one way or another it's looks like a bug, so it should either be created as empty array, returned as empty artray.

Something like:
[code]
 private function createStreamResource(
        RequestInterface $request,
        array $options,
        $context,
        &$http_response_header
    ) {
        $url = $request->getUrl();

        return $this->createResource(
            function () use ($url, &$http_response_header, $context) {
                if (false === strpos($url, 'http')) {
                    trigger_error("URL is invalid: {$url}", E_USER_WARNING);
                    return null;
                }
	        $http_response_header = array();
                return fopen($url, 'r', null, $context);
            },
            $request,
            $options
        );
    }

private function createResponseObject(
        RequestInterface $request,
        [b]?array[/b] $headers,
        TransactionInterface $transaction,
        StreamInterface $stream
    )    private function createResponseObject(
        RequestInterface $request,
        array $headers,
        TransactionInterface $transaction,
        StreamInterface $stream
    )