Cannot checkout documents from SharePoint 2010 repository using REST

Using Chemistry I can add a document to SharePoint 2010 via the AtomPub bindings. Adding the document gives me a Chemistry Document instance. When I call checkOut() on this instance, I get a "Bad Request" response (HTTP 400 status code), and the response body from SP 2010 is just the single word "documentId". The Chemistry exception is a CmisInvalidArgumentException.

Does anybody have working code to checkout documents from SP2010? I can't find any samples online.

Thanks,
Dave