Although the motet Nigra sum in the Libro primo de la fortuna (Rome, c. 1526) has long been attributed to Adrian Willaert, he is not its composer. No sixteenth-century source ascribes the piece to him. Rather, the attribution stems from a modern practice that this study terms the tacit attribution fallacy: the construal of apparent groupings of pieces in tables of contents as evidence of attributions. Two concordances for Nigra sum are found in manuscripts from the Veneto: the choirbook Verona 760, and the tenor partbook Bologna R142. Together, these indicate that the motet was originally composed for three voices, with two si placet voices added later. Implausible attributions for pieces such as Nigra sum illustrate the need for more careful attention to sixteenth-century composers’ work-lists. Avoiding the tacit attribution fallacy reveals how central sources such as the Libro primo evince a robust musical transmission.