public function getCapability($capabilityName)
    {
        if (empty($capabilityName)) {
            throw new \InvalidArgumentException('capability name must not be empty');
        }

        if (!$this->getRootDevice()->isCapabilityDefined($capabilityName)) {
            throw new \InvalidArgumentException('no capability named [' . $capabilityName . '] is present in wurfl.');
        }

        foreach ($this->modelDevices as $modelDevice) {
            /* @var Device\ModelDeviceInterface $modelDevice */
            $capabilityValue = $modelDevice->getCapability($capabilityName);
            if ($capabilityValue !== null) {
                return $capabilityValue;
            }
        }

        return null;
    }