A contractual arbitration award (lodo irrituale) and an expert determination (perizia contrattuale), having a contractual nature quoad effectum and performing the function of substituting the will of the parties, are challengeable exclusively on the grounds of defects capable of vitiating any expression of contractual will. The error relevant for the purposes of challenge must be substantial or essential and recognisable, consisting of a false representation or altered perception of the factual elements caused by having treated as established facts that certainly were not, and vice versa.