Tooltip Title format can not be combined with tick as categories

Using bar chart, and the next config:
...
axis: {
x: {
type: 'category',
categories: ['2010', '2011', '2012', '2013', '2014']
},
...
tooltip: {
format: {
title: function (v) {
return "Year " + v;
},
}
....
Without tooltip format, when event onfocus is triggered in the first bar of the chart the tooltip show for default "2010" in the title. But when using format, it shows "Year 0" when it should be "Year 2010".