Squid Proxy error for ftps

I'm running Squid v3.5.15 (installed on Windows Server 2012R2) as a simple relay to other external servers. Using Total Commander to connect, everything works for ftp, but fails for ftps. What have I missed? The access log reads:

1480977720.502      1 xxx.xxx.xxx.xxx TAG_NONE/400 3911 GET ftps://username:password@somewhere.com/ - HIER_NONE/- text/html

Thanks,

Chris