An employment contract does not belong to civil law transactions, therefore it cannot be invalidated. 

Regulation of labour relations by means of the analogy of the law of civil law contradicts Art. 5 OF THE LABOUR CODE. At the same time, the current legislation does not provide any grounds for the validity of a labour contract. Therefore, courts of general jurisdiction refuse to recognise such agreements as invalid both at the request of employees and at the request of employers. 

But arbitration courts have a different practice. 

You can read the full version of the article on the website of the legal portal ‘Pravo.ru’