How to perform calculation in DataGridView when a checkbox is ticked C#

• I have this case in C# Datagridview.

I want to calculate 3-Piece Average, on textbox shown above, whenever checkbox is checked.

I'm using this logic below:

 private void button1_Click(object sender, EventArgs e)
    {


        DataGridView data = this.qualitySetupDataGridView;
        try
        {
            int count = 0;
            double Sum = 0;

            for (int k = 0; k < data.Rows.Count - 1; k++)
            {

                if (data.Rows[k].Cells[5].Value != null)
                {
                    if (Convert.ToBoolean(data.Rows[k].Cells[5].Value) == true)
                    {

                        double EndsPerInch = double.Parse(data.Rows[k].Cells[4].Value.ToString());
                        Sum = Sum + EndsPerInch;
                        count++;
                        double Average = Sum / count;
                        textBox1.Text = Convert.ToString(Math.Round(Average, 2));

                    }

                }

                else
                {

                    textBox1.Clear();

                }
            }

• The code is working as it suppose to be.


• but first time, when a single checkbox is selected, I get Invalid Cast Exception Error.


• when a second checkbox is selected I again get Invalid Cast Exception Error.


• When third checkbox is selected I do not get Invalid Cast Exception Error.


• 
  

  and then the program works as it suppose to be.

  
  


• 
  

  The question here is this why it pops up invalid cast exception error in the beginning?.

  
  

is there a better way to achieve this?