Numeric user ID invalid in 7.3 ?

I am moving a postgres database onto an new machine, running the
current stable version of Postgres7.3 . I am finding broken application
software, and seem to have narrowed it down to a user id of 1001 .
That is one-thousand and one. I can create the user fine, put it
into a group, can grant all the permissions, but when the programs run,
or even if I try to execute : psql -d db -U 1001 , I am getting an
error message : ERROR: pg_class_aclcheck invalid user id 1001 .
If i create a new user id that is not numeric it seems to work fine,
but I am worried about having to convert the user ID used in all
the programs.

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