Service not found in dependency injection container when I run unit tests

Hey guys,

When I try to run my unit tests I have this error:

Phalcon\DI\Exception: Service 'collectionManager' was not found in the dependency injection container

The error occurs on method initialize of my \Phalcon\Mvc\Collection when I instantiate the object. Here's the code of my initialize method:

public function initialize()

{

$this->setConnectionService('MongoDB');    

}

Anyone knows how to solve that?