While preparing the flight, the crew asked for a modification to the flight plan due, in particular, to the risk of severe turbulence on the planned track. The new track meant that the crew would fly to the west of and alongside the air mass where the risk of severe turbulence was forecast. Shortly before reaching the en-route flight level, the crew asked the controller if turbulence had been reported in the sector. The controller replied that no turbulence had been reported.On approaching the air mass concerned by the SIGMET, the aeroplane entered a zone of turbulence. The suddenness and strength of the turbulence resulted in injury to two cabin crew members who were carrying out their customer-service duties.