Type mismatch. Value (x) does not match type number in column index 1

Question

I have google visualization pie chart that I'm drawing like so:

OIT_location_pie_chart1.setDataTable( google.visualization.data.group(
            // get the filtered results
            OIT_location_pie_chart.getDataTable(), [1], [{'column': 15, 'aggregation': google.visualization.data.sum, 'type': 'number'}] //problem at 'type': 'number'
            ));
            OIT_location_pie_chart1.draw();
      });

There are some column values in the table that are empty after a certain date, so that data errors out if the user tries to show data after that specific date.

How do I check the data type for the rows that are erring out and account for them/ignore them?