What needs to be done in order to avoid the following error from a operation event handler?

 org.nuxeo.ecm.core.storage.sql.coremodel.SQLDocumentVersion$VersionNotModifiableException: Cannot set property on a version:

My code works correctly when documents are created from the UI – it sets additional metadata based on web service call results, similar to the external web service example on the Nuxeo blog.

However when I attempt to use cmislib's createDocument call, the VersionNotModifiableException error is returned when Document.Save is called on the modified object.

Is there something I should be doing in my Operation, or some special property or set of properties I should be setting on the createDocument call?