Also, the type argument selector has to be Snowball, not snowball.
This is important because Minecraft is case-sensitive.
That's also the reason why the command block wasn't working,
it just didn't detect the entity "snowball" and it shows the latest output, in this case
the output which was generated when the command block was activated and the
command block, without a command in it, gave that output.
Confirmed for Minecraft 1.9 pre3
Confirmed for 16w20a.