alert auto closes even with autoClose=false

I am on Ent v12 working to get to v14 (hopefully by end of next month).

I have a simple alert in my controller:

app.alert.show('case_quote_exists',{
  level: 'confirmation',
  messages: 'The selected Quote is already related to case '+ case_number,
  autoClose: 'false',
  onConfirm: function(){
    alert("Confirmed!");
  },
  onCancel: function(){
    alert("Cancelled!");
  }
});

But the alert popup closed before one can make a choice, in spite of the autoClose: 'false'.

Any thoughts? 

Thanks,
FrancescaS