src/Storage/Configurator/AbstractConfigurator.php
public function assertString($config, $key)
    {
        if (array_key_exists($key, $config) && !is_string($config[$key])) {
            throw new ConfigurationException($key." must be a string.");
        }
        return $this;
    }
public function assertString($config, $key)
    {
        if (array_key_exists($key, $config) && !is_string($config[$key])) {
            throw new ConfigurationException($key." must be a string.");
        }
        return $this;
    }
Assert the property is string or is not set
@param array $config Array being validated
@param string|int $key Inspected array key
@return $this
@throws ConfigurationException On a failed assertation