protected function checkInstances()
    {
        $output = [];
        foreach ($this->checks as $check) {
            list($checkClass, $checkTitle) = $check;
            if (is_string($checkClass)) {
                $checkInst = Injector::inst()->create($checkClass);
                if ($checkInst instanceof EnvironmentCheck) {
                    $output[] = [$checkInst, $checkTitle];
                } else {
                    throw new InvalidArgumentException(
                        "Bad EnvironmentCheck: '$checkClass' - the named class doesn't implement EnvironmentCheck"
                    );
                }
            } elseif ($checkClass instanceof EnvironmentCheck) {
                $output[] = [$checkClass, $checkTitle];
            } else {
                throw new InvalidArgumentException("Bad EnvironmentCheck: " . var_export($check, true));
            }
        }
        return $output;
    }