Liquibase preconditions not working

I'm trying to use liquibase to track changes to a postgresql database using dropwizard-migrations. I'd like to be able to run the migration on the existing production database instead of rebuilding from scratch. Right now I'm testing in staging. I've created a changeset with a precondition.

<changeSet id="3" author="me"> <preConditions onFail="CONTINUE"> <not> <sequenceExists sequenceName="emails_id_seq"/> </not> </preConditions> <createSequence sequenceName="emails_id_seq" startValue="1" incrementBy="1" /> </changeSet>

My goal is to skip applying the changeset if the sequence is already there. Seems straightforward, but it's not working.

ERROR [2013-09-13 22:19:22,564] liquibase: Change Set migrations.xml::3::me failed. Error: Error executing SQL CREATE SEQUENCE emails_id_seq START WITH 1 INCREMENT BY 1: ERROR: relation "emails_id_seq" already exists ! liquibase.exception.DatabaseException: Error executing SQL CREATE SEQUENCE emails_id_seq START WITH 1 INCREMENT BY 1: ERROR: relation "emails_id_seq" already exists

I've tried using the MARK_RAN instead of CONTINUE too. No luck with that.

(Cross posted at Stack Overflow)