ConcurrentModificationException from SqlGeneratorFactory.getGenerators

I am using Liquibase via its java API. Periodically, I get this error:

Caused by: java.util.ConcurrentModificationException

at liquibase.Liquibase.update(

        at …

Has anyone else come across this? I’m new to Liquibase and taking a quick look at the code, I didn’t see any obvious synchronization problem.

I have one custom generator (a workaround for DB2:, which I register with the SqlGeneratorFactory before creating an instance of Liquibase.

I haven’t seen it, and I’m not seeing any code in liquibase that would be attempting to modify the generators list during the getGenerators() call. 

SqlGeneratorFactory is a singleton. Is there a different thread that would be trying to start up liquibase when you are updating with a different thread?