Trying to blackhole or reject attacking IPs with route fails

My server is under heavy attack from an ip group in Hong Kong.

Trying to add a blackhole route in Yosemite with any of these commands or similar:

sudo route -v add -net 103.41.124.0/24 -blackhole

sudo route -v add -net 103.41.124.0/24 -reject

sudo route -v add -host 103.41.124.48 -reject

results in something like:

sudo route -v add -net 103.41.124.0/24 -blackhole

Password:

u: inet 103.41.124.0; RTM_ADD: Add Route: len 116, pid: 0, seq 1, errno 0, flags:<UP,GATEWAY,STATIC,BLACKHOLE>

locks: inits:

sockaddrs: <DST,NETMASK>

103.41.124.0 (0) 0 ffff ff

route: writing to routing socket: Invalid argument

add net 103.41.124.0: Invalid argument

What's the problem here?