System.InvalidCastException: Object cannot be cast from DBNull to other types.?

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C#

ASP.NET

C#4.0

hi can any one help me to solve this..
am not trying to solve this

here is my code
decimal grdTotal=0;

protected void GridView1_RowDataBound1(object sender, GridViewRowEventArgs e)
    {


        if (e.Row.RowType == DataControlRowType.DataRow)
        {
            decimal rowTotal = Convert.ToDecimal
                (DataBinder.Eval(e.Row.DataItem, "tsMonday"));

            grdTotal = grdTotal + rowTotal;

        }

        if (e.Row.RowType == DataControlRowType.Footer)
        {
            Label lbl = (Label)e.Row.FindControl("lblTotal");
            lbl.Text = grdTotal.ToString("c");
        }




    }

decimal rowTotal = Convert.ToDecimal

this line am getting error..

please suggest me..
thank you