curl query to check out a document using CMIS

I am trying to checkout a document using the REST-based CMIS protocol, but I get an error from the server (Alfresco).

Am I misusing curl?Or am I missing something in my request?

curl --user admin:admin -F "atomentry=@atomentry.xml" http://localhost:8080/alfresco/service/cmis/checkedout

With atomentry.xml being:

<?xml version="1.0" encoding="utf-8"?><entry xmlns="http://www.w3.org/2005/Atom"xmlns:cmisra="http://docs.oasis-open.org/ns/cmis/restatom/200908/"xmlns:cmis="http://docs.oasis-open.org/ns/cmis/core/200908/"><cmisra:object><cmis:properties><cmis:propertyId propertyDefinitionId="cmis:objectId"><cmis:value>workspace://SpacesStore/3e13d089-39cf-48a4-b0b6-773b602bbcc0</cmis:value></cmis:propertyId></cmis:properties></cmisra:object></entry>