Cannot call method 'instance' of undefined

Question

When using the following code;

var pvChart = new pv.Panel();
pvChart.width(200);
pvChart.height(200); 

var pvBar = pvChart.add(pv.Bar);
pvBar.data([1,2,3]);

console.log(pvBar.fillStyle());

I get the error;

Cannot call method 'instance' of undefined

It refers to the pvBar.fillStyle(). Using this I want to retrieve the bars fillStyle for later use. Can anyone tell me the reason of this error and how to solve this?