PHP 7.1.2 Generator Segfault

<?php
class Test {
    public function run() {
        $this->html = new HtmlHelper();
        $this->test = new TestHelper();

        $test = (string)$this->html('main', function() {
            $category = [ 'name' => 'test', 'philosophy' => 'Paragraph about philosophy of working with brands.' ];

            for($i = 0; $i < 50; $i++) {
                yield $this->html('article', function() use($category) {
                    yield $this->html('h3', $category['name']);
                    yield $this->html('div', $this->html->lower($category['philosophy']));

                    yield $this->html('div', $this->test('This is a test'));
                    yield $this->test->doThing();
                });
            }
        });
        die($test);
    }

    public function __call($name, $args) {
        $helper = $this->{$name};
        return $helper(...$args);
    }
}

class HtmlHelper {
    public function __invoke($name, $content) {
        return '<'.$name.'>'.$this->_renderChild($content).'</'.$name.'>';
    }

    protected function _renderChild($value) {
        if(is_callable($value) && is_object($value)) {
            return $this->_renderChild($value($this));
        }

        if(is_array($value) || $value instanceof \Generator) {
            $output = '';

            foreach($value as $part) {
                $output .= $this->_renderChild($part);
            }

            return $output;
        }

        return (string)$value;
    }

    public function lower($value) {
        return strtolower($value);
    }
}

class TestHelper {
    public function __invoke($value) {
        return strtoupper($value);
    }

    public function doThing() {
        return 'other thing';
    }
}

(new Test())->run();