google.visualization.PieChart is not a constructor

I am using pie chart to show the data here is my code I have added the following script

<script type="text/javascript" src="https://www.gstatic.com/charts/loader.js"></script>

    <script type="text/javascript">
        google.charts.load('current', { 'packages': ['corechart', 'gauge'] });

and my function which is used to display chat:-

self.DrawServiceGraph = function (obj) {
                var dataArray = new Array();
                dataArray.push(['Services', 'Total']);
                var subCat = ko.utils.arrayFirst(self.SelectedServiceSubCategoryList(), function (o) { return o.Id === obj.Id; });
                if (subCat !== undefined) {
                    $.each(subCat.SSC_List(), function (index, item) {
                        var value = parseInt(item.Value());
                        dataArray.push([item.SSC_Name, value]);
                    });

                    var data = google.visualization.arrayToDataTable(dataArray);

                    var options = {
                        title: '', height: 230, width: 330,
                        chartArea: { left: 0, width: '100%' },
                        legend: { position: 'right', alignment: 'start', maxLines: 5, textStyle: { fontSize: 10 } }
                    };

                    var chart = new google.visualization.PieChart(document.getElementById('Services-chart' + obj.Id));
                    chart.draw(data, options);
                }
            }

Then i got the error from line:

google.visualization.Pie Chart is not a constructor and google.visualization.arrayToDataTable is not a constructor

What should be the possible solutions to avoid this errors?

are you waiting for the load statement to finish?

use the callback to know when it is ok to begin drawing...

google.charts.load('current', {
  callback: function () {
    // begin drawing
  },
  packages: ['corechart', 'gauge']
});

-- or --

google.charts.load('current', {
  packages: ['corechart', 'gauge']
});
google.charts.setOnLoadCallback(function () {
  // begin drawing
});