A fastener device for closing a fluid reservoir (1) having a neck (10) with outer peripheral reinforcement (11) that co-operates with the neck to define a bottom shoulder (12). The fastener device includes a fastener ring (2) for holding a closure member (4) on the neck (10), and which includes a peripheral skirt (21) extending around the neck below the bottom shoulder (12). The skirt (21) is provided with fastener elements (26a, 26b) forming contact zones (27a, 27b) for coming into engagement with the neck (10) below the bottom shoulder (12). A blocking hoop (3) is provided that is engaged around the peripheral skirt (21) in a final mounted position. The peripheral skirt (21) forms vertical blades (22) separated by axial slots (23), the blocking hoop (3) forming vertical branches (33) that are arranged in the axial slots (23) between two vertical blades (22), in the mounted position.
