pg_dumpall: query failed:

I have this problem with a nightly script that dumps the db:

pg_dumpall: query failed: ERROR:  Unable to convert abstime 'invalid' to
timestamp
pg_dumpall: query was: SELECT usename, usesysid, passwd, usecreatedb,
usesuper, CAST(valuntil AS timestamp) FROM pg_shadow WHERE usesysid <>
(SELECT datdba FROM pg_database WHERE datname = 'template0');

Postgres version is 7.3.2.

Any hints?

ciao
Guido