The present paper examines one of the classical concepts of civil law: the agreement upon the form of statements of will (pactum de forma). According to the article 76 of the Polish Civil Code, parties to an agreement may stipulate that they will take legal actions (exactly: make statements of will) in a particular form (e.g. in writing or in notary form). Polish civil law also provides a general rule that an act which fails to comply with such a formal requirement 'produces no effect'. This expression, crucial for the proper understanding of the whole regulation, is, however, not explicit enough and leads to severe controversies. According to certain authors, such a contract is only ineffective, whereas according to others, it is null and void. The present article aims to find a solution to this problem by applying the theory of conventional acts. In the light of this theory, statements of will and other legal acts have to be done in compliance with the conventional rules imposed by legal provisions, similarly to other conventional acts, which should follow conventional rules of different nature (like some acts of social behaviour, e.g. the way people greet each other). If these requirements are not observed, the whole act cannot be recognized as an act of the particular type. Consequently, the form prescribed by the parties for their future agreements should also be considered a conventional rule which is a condition for validity of the entire contract. Such agreement is null and void, but not nonexistent, since the concept of 'nonexistent legal act' (negotium non existens), seems to be too vague and uncertain to be used to describe defectiveness of civil contracts.