Adding a Profile Taxonomy selection field to profile causes fatal Drupal error

After installing profile_taxonomy (I had to modify profile_taxonimy.install - see http://drupal.org/node/810950 ), I tried to create a Taxonomy Profile selection field.

When creating or modifying this field, a fatal PHP error occurs which breaks Drupal until I manually edit the profile_taxonomy table:

Fatal error: Cannot access empty property in /srv/www/webinars02-drupal-6.16/sites/all/modules/profile_taxonomy/profile_taxonomy.module on line 268

It looks as though the wrong value for fid is being inserted into the table:

mysql> select * from profile_taxonomy;
+-----+-----+
| fid | vid |
+-----+-----+
|   0 |   1 | 
+-----+-----+
1 row in set (0.00 sec)

If I manually change fid from '0' to '2', Drupal recovers and the new Profile Taxonomy field is usable in user profiles: selectiion, views, etc.

If I go back and modify the field via /admin/user/profile. Drupal again crashes with the same fatal error message, and I find another line has been added to the table, again with fid = '0'. Removing this line restores Drupal to life.

It looks to me as if there is a problem near line 110 of profile_taxonomy.module which means profile_taxonomy_add() is receiving the incorrect $field parameter when it's called.