Undefined offset: 2 in drupal_http_request()

I have just upgraded from Drupal 6 to 7, and configured backup_migrate and backup_migrate_files. When I run either the database or public files backup I get the following message in the log:

Notice: Undefined offset: 2 in drupal_http_request() (line 993 of /home/sites/letchwortharts.org/public_html/test/includes/common.inc).

This is repeated 6 times (4 with offset 1, 2 with offset 2), followed by a message that the backup was successful. A backup file is created, but I have not tried restoring from it yet.

It may not be relevant, but the server I am using does not support Drupal issuing HTTP requests to itself, though it does so happily to other destinations.

Please advise whether this is likely to affect the integrity of the backups.

With thanks,
Tony