/playsound command not working when 2 or more players were targeted by the command

I have a server of 1.9, but the command of the /playsound didn't work when multi-ply players has been selected, (when SuccessCount is 2 or more)
if I doing a command such as:
/playsound minecraft:block.anvil.land master @a 30
(when 2 players or more are online and affected by the command)
this thing happened after the change of the source.

Error Json Text: That Player cannot be found