The subject of this study is to assess the legal nature of the obligations imposed on the entity managing the register of shareholders of a non-public joint-stock company in case of the entity's justified doubts regarding the legality and authenticity of the documents justifying the entry into the register. The starting point for the considerations undertaken is the tension visible in the Commercial Companies Code between the sparse code regulation of the obligation to resolve reasonable doubts and the extensive nature of the function that the legislator assigns to the entity managing the register of shareholders in the proceeding for the entry into the register, which seems to go beyond the narrowly understood registration function. The research on the scope of the obligation to resolve justified doubts in the Code, as well as the practice of applying this obligation by entities managing registers of shareholders, justify the conclusion that this practice is contrary to the legislator's intentions, and the function that the legislator assigns to the entity managing the register of shareholders is not implemented in practice.