getSublistFields doesn't return list of fields

I am trying to get a list of 'calls' associated with a Support Case record via a Workflow Action Script.
getSublists returns a list of sublist that includes 'calls'.  However, I can't get to 'calls'. 

What am I missing?

Code snippet:
       var rec = record.load({
            type: record.Type.SUPPORT_CASE,
            id: scriptObj.getParameter({name: 'custscript3'}),
            isDynamic: true
        });

        var mySublists = rec.getSublists();       
        log.debug({
            title: 'Sublists:',
            details: mySublists
            });

        var sublistFields = rec.getSublistFields({  //  This is line #38
            sublistId: 'calls'
       });
        log.debug({
            title: 'Sublist Fields:',
            details: sublistFields
            });




The log shows:
Sublists:    ["recmachcustrecord_bako_case_no","messages","calls","usernotes","activities","mediaitem","tasks","events","transactions","escalatehist","escalateto","systemnotes","activeworkflows","workflowhistory"]