DatabaseError: no such index: trial_instances_ami_id

When I try to run the manage.py migrate command, I'm getting an error:

python manage.py migrate
Running migrations for adminaudit:
 - Migrating forwards to 0002_auto__chg_field_auditlog_username__chg_field_auditlog_representation__.
 > adminaudit:0001_initial
 > adminaudit:0002_auto__chg_field_auditlog_username__chg_field_auditlog_representation__
 - Loading initial data for adminaudit.
No fixtures found.
Running migrations for trial:
 - Migrating forwards to 0005_auto__add_field_gmailalias_user.
 > trial:0001_initial
 > trial:0002_auto__add_field_userprofile_initial_ip
 > trial:0003_allow_unreserved_instances
 > trial:0004_auto__add_gmailalias
Traceback (most recent call last):
  File "manage.py", line 16, in <module>
    execute_manager(settings)
  File "/Users/nateaune/Dropbox/code/venvs/awstrial/lib/python2.7/site-packages/django/core/management/__init__.py", line 438, in execute_manager
    utility.execute()
  File "/Users/nateaune/Dropbox/code/venvs/awstrial/lib/python2.7/site-packages/django/core/management/__init__.py", line 379, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/Users/nateaune/Dropbox/code/venvs/awstrial/lib/python2.7/site-packages/django/core/management/base.py", line 191, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "/Users/nateaune/Dropbox/code/venvs/awstrial/lib/python2.7/site-packages/django/core/management/base.py", line 220, in execute
    output = self.handle(*args, **options)
  File "/Users/nateaune/Dropbox/code/venvs/awstrial/lib/python2.7/site-packages/south/management/commands/migrate.py", line 105, in handle
    ignore_ghosts = ignore_ghosts,
  File "/Users/nateaune/Dropbox/code/venvs/awstrial/lib/python2.7/site-packages/south/migration/__init__.py", line 191, in migrate_app
    success = migrator.migrate_many(target, workplan, database)
  File "/Users/nateaune/Dropbox/code/venvs/awstrial/lib/python2.7/site-packages/south/migration/migrators.py", line 221, in migrate_many
    result = migrator.__class__.migrate_many(migrator, target, migrations, database)
  File "/Users/nateaune/Dropbox/code/venvs/awstrial/lib/python2.7/site-packages/south/migration/migrators.py", line 292, in migrate_many
    result = self.migrate(migration, database)
  File "/Users/nateaune/Dropbox/code/venvs/awstrial/lib/python2.7/site-packages/south/migration/migrators.py", line 125, in migrate
    result = self.run(migration)
  File "/Users/nateaune/Dropbox/code/venvs/awstrial/lib/python2.7/site-packages/south/migration/migrators.py", line 99, in run
    return self.run_migration(migration)
  File "/Users/nateaune/Dropbox/code/venvs/awstrial/lib/python2.7/site-packages/south/migration/migrators.py", line 81, in run_migration
    migration_function()
  File "/Users/nateaune/Dropbox/code/venvs/awstrial/lib/python2.7/site-packages/south/migration/migrators.py", line 57, in <lambda>
    return (lambda: direction(orm))
  File "trial/migrations/0004_auto__add_gmailalias.py", line 19, in forwards
    db.delete_index('trial_instances', ['ami_id'])
  File "/Users/nateaune/Dropbox/code/venvs/awstrial/lib/python2.7/site-packages/south/db/generic.py", line 689, in delete_index
    self.execute(sql)
  File "/Users/nateaune/Dropbox/code/venvs/awstrial/lib/python2.7/site-packages/south/db/generic.py", line 150, in execute
    cursor.execute(sql, params)
  File "/Users/nateaune/Dropbox/code/venvs/awstrial/lib/python2.7/site-packages/django/db/backends/sqlite3/base.py", line 234, in execute
    return Database.Cursor.execute(self, query, params)
django.db.utils.DatabaseError: no such index: trial_instances_ami_id