Counterbalancing with Batch Session Data

Hello! I'm looking to assign participants to two versions of a task by their order of entry, using batch session data. The code seems to work since Version A and Version B would alternate when I use the same General Multiple link again and again. However, for the first run, the console says "Can send only one batch session patch at a time", and the "counterbalance" counter would be 1 and not 2 on the second run.

Here is the code (the first few lines are probably where the issue is from):

    /*start experiment, the JATOS version*/
    jatos.onLoad(() => {


      //use batch session data to do counterbalancing
      let cb = jatos.batchSession.get("counterbalance"); //retrieve the data
      console.log("cb initial = "+cb)
      if(typeof cb == "undefined"){ 
        //deal with the first ever member of the batch?
        cb = 1; //cb defines if the participant is the nth one in batch
        jatos.batchSession.add("/counterbalance", cb);
      }
      current_cb = cb;
      console.log("current_cb = " + current_cb)
      jatos.batchSession.set("counterbalance", cb+1); //add the counter

    /* initialize jsPsych */
    var jsPsych = initJsPsych({
      on_trial_start: jatos.addAbortButton,
      on_trial_finish:function(){
      let trial_data_save = jsPsych.data.get().last(1).json();//retrieve data from trial
          jatos.appendResultData(trial_data_save); //adds data to server as the trial goes
      },
      on_finish: () => jatos.endStudy() //end
    });


    //IMPORTANT: for setting counterbalancing for the rest of the task
    let counterbalance_main = current_cb % 2;
    
    /* task */
    let timeline = [];
    let stimulus = {
        type: jsPsychHtmlButtonResponse,
        choices: ['Continue'],
        stimulus: function(){
            //console.log(jatos.batchSession.get)
            let stim;
            if(counterbalance_main == 0){
                stim = `Version A`
            } else if (counterbalance_main == 1){
                stim = `Version B`
            }
            return stim
        }
    }
    timeline.push(stimulus);
    jsPsych.data.addProperties({
        ptp_cb: current_cb
    });
    jsPsych.run(timeline);


  });

As a beginner in JavaScript and JSON, this might be because I did not really understand the batch session functions 🙈, so perhaps there would be a rather easy fix.

Many thanks for your help!