Error in trying to dump any database

I have a 7.1.2 system which gives the following error on any pg_dump:

DumpComment: SELECT failed: 'ERROR:  dtoi4: integer out of range
'.

This happens with any database.  E.g. I made a very tiny database
with a single table and two records and the same error and message
is obtained.

As far as I can tell, all of the data is intact, queries work, etc.
Any ideas?

--Martin