[CentOS] Firewalld zone conflict

Hi,
 In CentOS7 I have added two zones using firewall-cmd, each for
different ports/services and interfaces when I try to --add-source to
the second zone with an IP that is already in the first I am getting an
'Error: ZONE_CONFLICT' message. Am I trying to do something illogical or
is this a feature?

Cheers,
Colin.