dpkg - Why does apt not display an error message even if flock is used?

Running script

#!/bin/bash
(
 flock 9 
  # ... commands executed under lock ...
 fuser -v /var/lib/dpkg/lock
 apt-get -f --assume-no install
) 9>/var/lib/dpkg/lock

as superuser does not display an error message. But if there is e.g. synaptic running,apt-get will display an error message:
"E: Could not get lock /var/lib/dpkg/lock - open (11: Resource temporarily unavailable)".