It was returning
Examples:
> bitcoin-cli setban "192.168.0.6" "add" 86400
> bitcoin-cli setban "192.168.0.0/24" "add"
> curl --user myusername --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "setban", "params": ["192.168.0.6", "add" 86400] }' -H 'content-type: text/plain;' http://127.0.0.1:8332/
(Missing the comma on the RPC example)