1.5->1.6 db update script for mysql 4 fails

When I run the 1.5 to 1.6 update script for php4, I get an error on the following line:
ALTER TABLE `civicrm_activity` DROP INDEX `FKEY_activity_type_id`;
It seems the index to be dropped doesn't exist. Inspecting the table, the FKEY_ prefix doesn't seem to have been used.