postgres subquery uses ungrouped column

I have a query in which I am trying to add a column which contains a percentage of records which have a certain value versus the total number of records. I can get the subquery to work on its own, but when I plug it into the main query it fails with an error : ERROR: subquery uses ungrouped column "ipam_addresses.host_id" from outer query

my query is:

select 
    ipam_networks.ipam_id, 
    ipam_networks.net_name, 
    ipam_networks.net_cidr, 
    zone, 
    count(*), 
    to_char(100*(select count(*) where ipam_addresses.host_id = 0) / (select count(*)), '999D99%') v 
from 
    ipam_addresses 
left outer join 
    ipam_networks 
    on ipam_addresses.parent_id = ipam_networks.ipam_id 
where 
    ipam_addresses.ip_state != 'DHCP_FREE' 
group by  
    ipam_networks.ipam_id, ipam_networks.net_name, ipam_networks.net_cidr;

Why is it throwing this error even if I specify the table name in the subquery, and what can I do to fix this?