[Solved] ASP.Net GridView Column Total Error: Input string was not in a correct format when null

i am trying to display gridview and total in footer, but have some null value in column.

so how to avoid this 

Input string was not in a correct format.

Label lblrecamt = (Label)e.Row.FindControl("Rec_Amt");
lblTotalAMt += Convert.ToDecimal(lblrecamt.Text);

Decimal lblTotalAMt = 0;
Decimal TotalValueSA = 0;
protected void gvsummaryrpt_RowDataBound(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.DataRow)
    {
        Label lblSA = (Label)e.Row.FindControl("Total_Amt");
        TotalValueSA += Convert.ToDecimal(lblSA.Text);
 
        Label lblrecamt = (Label)e.Row.FindControl("Rec_Amt");
        lblTotalAMt += Convert.ToDecimal(lblrecamt.Text);
    }
    else if (e.Row.RowType == DataControlRowType.Footer)
    {
        Label lbSA = (Label)e.Row.FindControl("lblTotalSA");
        lbSA.Text = TotalValueSA.ToString();
 
        Label lbrec = (Label)e.Row.FindControl("lblTotalAMt");
        lbrec.Text = lblTotalAMt.ToString();
    }
}