Version 4.0.0 stores NULL values in the colum:
ezurlalias_ml.alias_redirects

So the query:
ALTER TABLE ezurlalias_ml ALTER COLUMN alias_redirects SET NOT NULL;

in the update file: update/database/postgresql/4.0/dbupdate-4.0.0-to-4.0.1.sql

doesn't work properly, because it meets NULL values in the table.