domain mapping + www prefix not working, works OK without www.

Hi,

I installed Wordpress Multisite onto a site with the www. prefix today.

(using the sub-folder method, softaculous auto picked this)

When adding a Custom Domain in the child site domain mapper it works fine if I add a domain but if I use a www. prefix + the domain it fails to detect the site.

It just redirects to the home page of the master site.

Should I have not used www. in the master main site install? Or can this be fixed.

Thanks