 After Willibrord's death the diocese suffered greatly from the incursions of the Frisians, and later on of the Vikings.Whether Willibrord could be called the first bishop of Utrecht is doubtful; as James Palmer points out, "there was no real concept of a well-defined bishopric until at least the days of Alberic (775–84)".And while Saint Boniface is referred to in his hagiographies as the successor of Willibrord (and, in turn, Gregory of Utrecht is referred to as the successor to Willibrord and Boniface), this does not necessarily mean "successor as bishop", but rather that they succeeded each other as missionaries to the Frisians.