Debian "dump" problem

Hi,

What does "dump" refer to in this message? I've deleted any databases
i had.

apt-get install postgresql

Reading Package Lists... Done
Building Dependency Tree... Done
Suggested packages:
   libpgperl libpgtcl postgresql-dev pgaccess python-pygresql python-psycopg
python-popy python-pgsql pgdocs
The following NEW packages will be installed:
   postgresql
0 upgraded, 1 newly installed, 0 to remove and 565 not upgraded.
1 not fully installed or removed.
Need to get 0B/3596kB of archives.
After unpacking 9196kB of additional disk space will be used.
Preconfiguring packages ...
(Reading database ... 86561 files and directories currently installed.)
Unpacking postgresql (from .../postgresql_7.4.1-3_i386.deb) ...
You must have an up to date dump before upgrading to PostgreSQL 7.4
dpkg: error processing /var/cache/apt/archives/postgresql_7.4.1-3_i386.deb
(--unpack):
  subprocess pre-installation script returned error exit status 1
Errors were encountered while processing:
  /var/cache/apt/archives/postgresql_7.4.1-3_i386.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)