can't find package Pextlib 1.0

I've just installed DarwinPorts 1.3.1 on my MacBook Pro (with OS  
10.4.8). When I try to run 'port' from a terminal, I get:

can't find package Pextlib 1.0
     while executing
"package_native require Pextlib 1.0"
     ("eval" body line 1)
     invoked from within
"eval package_native $args"
     (procedure "package" line 14)
     invoked from within
"package require Pextlib 1.0"
     (procedure "dportinit" line 311)
     invoked from within
"dportinit ui_options global_options global_variations"
Error: /opt/local/bin/port: Failed to initialize ports system, can't  
find package Pextlib 1.0

Can anyone help me with this?

Malcolm