Callback handler passed to getMembers() not being called

I'm using 2.4, and trying to use getMembers to  access hierarchy information. I'm using an OLAP data source. The example contained in the documentation does not work for me: https://www.flexmonster.com/api/getmembers/
 
I tried using that code directly (with the name of my hierarchy), and while the callback works as expected if I define a function in the getMembers call, using the specified syntax does not invoke the callback.
 
So the following is not working for me: 

flexmonster.getMembers(currentReportHierarchy, '', 'onGetMembers');

function onGetMembers(members) {
 for (var i = 0; i < members.length; i++) {
 console.log(members[i]);
 }
}

Were there changes made to this function between 2.4 and 2.6?