cmisapi getObject fails.

In my drupal code I retrieve a list of documents in a folder in alfresco using cmisapi_getChildren(). With the objectId for one of the children found, which is a document in the folder, I use

cmisapi_getObject($repositoryId, $objectId); 

In wireshark I can see that the url generated is

localhost:8080/alfresco/s/cmis/s/workspace:SpacesStore/arg/p?path=workspace://SpacesStore/da191e7e-77c2-46e8-8caa-ee36d067dff0&filter=&includeAllowableActions=&includePolicyIds=&includeRelationships=&includeACL=&renditionFilter=

which returns a 404 error. Is it ok that the string /workspace:SpacesStore occurs twice? I don't know what's wrong in the url, why it would generate a 404.

Using

cmis_service('default', $documentUrlPrefix.$objectId, $get_props); 

I can retrieve the document properly.