allow-transfer with distinct IP rejected

[Dr. Lars Hanke]

I have 2 Bind9.9.5 as of current Debian Jessie running is a Master/Slave 
setup. Everything works fine, if I put

allow-transfer { 172.16/16; };

on the master. If I want to be more restrictive, i.e. specify the IP of 
the slave immediatey

allow-transfer { 172.16.11.35; };

I see logs like

transfer of '178.168.192.in-addr.arpa/IN' from 172.16.10.16#53: failed 
while receiving responses: REFUSED

What am I doing wrong?

Thanks for your help,
  - lars.