No name resolution when slave is down

[Dennis J.]

Hi,
This morning the slave in our nameserver setup went down and surprisingly 
none of the domains hosted on these system could be resolved anymore even 
with the master working perfectly fine.
When I send queries directly to the master it resolves the domains fine so 
I'm not sure why a failure of the slave leads to a total failure of the 
service.
Does anyone have an idea what might cause this behavior?

Regards,
   Dennis