the /playsound command does not work with @a when >1 player passes the query

The /playsound command doesn't work with multiple players, meaning when you use @a with parameters that limit it to one person, it works, however when more than one player qualifies it fails.

how to reproduce
Two people on a server
One types /playsound random.explode @a[r=1], should work assuming they are more than 1 block apart.
type /playsound random.explode @a, should fail