App configuration used:

                 'log'=> array (
            array(
                'writerName' => 'Stream',
                'writerParams' => array(
                    'stream' => APPLICATION_PATH.'/production.log',
                ),
                'formatterName' => 'Simple',
                'formatterParams' => array(
                    'format' => '%timestamp%    %priorityName%  %sessionid% %user%: %message%'.PHP_EOL,
                ),
                'filterName'   => 'Priority',
                'filterParams' => array(
                    'priority' => 4,        // warn
                ),
            ),
        ),

Result:

{quote} Fatal error: Uncaught exception 'Zend\Log\Exception\InvalidArgumentException' with message 'Writer must be an instance of Zend\Log\Writer or you should pass a configuration array' in /path/library/Zend/Log/Logger.php on line 484 {quote}

Because Zend\Log\Logger expects and array(array('name'=>'val')) configuration, but PluginBroker flattens the array. The flattening occurs in Zend/Loader/PluginBroker.php:191

        } else {
            $r = new \ReflectionClass($class);
            $instance = $r->newInstanceArgs($options);
        }

When arriving at this function, $options contains array(array()). Then instance is created with just array() because of ReflectionClass::newInstanceArgs() (see http://php.net/manual/en/…).

Fix:

 $instance = $r->newInstanceArgs(array($options));

Workaround: Use a tripple-nested config for log, ie:

  'log' => array(array(array('writerName'=> ... ))),