§1. The following are admitted to the novitiate invalidly: 1/ one who has not yet completed seventeen years of age; 2/ a spouse, while the marriage continues to exist; 3/ one who is currently bound by a sacred bond to some institute of consecrated life or is incorporated in some society of apostolic life, without prejudice to the prescript of can. 684; 4/ one who enters the institute induced by force, grave fear, or malice, or the one whom a superior, induced in the same way, has received; 5/ one who has concealed his or her incorporation in some institute of consecrated life or in some society of apostolic life.
§2. Proper law can establish other impediments even for validity of admission or can attach conditions.
Source: Code of Canon Law (Vatican.va)