Mendix SDK branch not working?

When I try to use the Mendix SDK on a branch the changes in the script are comitted to the main line. Is this a bug? I'm using the code:

const username = process.argv[2];
const apikey = process.argv[3];

const projectId = process.argv[4];
const projectName = process.argv[5];
const moduleName = process.argv[6];
const pageName = process.argv[7];

let branchName = null; // null for mainline
if (process.argv[8]){
      branchName = process.argv[8];
}

let revNo = -1; // -1 for latest
if (process.argv[9]){
      revNo = process.argv[9];
}

const client = new MendixSdkClient(username, apikey);
const project = new Project(client, projectId, projectName);

client.platform().createOnlineWorkingCopy(project, new Revision(revNo, new Branch(project, branchName)))
    .then(workingCopy => loadDomainModel(workingCopy))
    .then(workingCopy => {
        ...
    })
    .then(workingCopy => workingCopy.commit())
    .done(
	revision => console.log(`Successfully committed revision: ${revision.num() }. Done.`),
	error => {
		console.log('Something went wrong:');
		console.dir(error);
	});

(I also tried setting branchName hardcoded)