acme.sh script to include the webmail subdomain on a shared (Namecheap) server

For a long time I have used acme.sh to create 90-day SSL certs for my example.com domain. Never had a problem.

However, I now wish to start using the webmail interface (webmail.example.com - included with hosting), but the usual acme.sh script fails.

This works:

./acme.sh --issue -d example.com -d www.example.com -w /home/myuserid/public_html

This fails:

./acme.sh --issue -d example.com -d www.example.com -d webmail.example.com -w /home/myuserid/public_html

What must I do to fix this? My webhost is Namecheap.