An error occurred during update row value to add alphabet in column value

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C#

ASP.NET4

protected void RowUpdating(object sender, GridViewUpdateEventArgs e)
{
GridViewRow row = Gridview1.Rows[e.RowIndex];
string pan = (row.FindControl("txtPan") as TextBox).Text;
string Depository = (row.FindControl("txtDepository") as TextBox).Text;
string Mobile = (row.FindControl("txtMobile") as TextBox).Text;
string Email = (row.FindControl("txtEmail") as TextBox).Text;
string DPID = (row.FindControl("txtDPID") as TextBox).Text;
string Client_Bef_Id = (row.FindControl("txtClient_Bef_Id") as TextBox).Text;
string UPI_ID = Convert.ToString((row.FindControl("txtUPI_ID") as TextBox).Text);
DataTable dt = ViewState["dt"] as DataTable;
dt.Rows[e.RowIndex]["Pan"] = pan;
dt.Rows[e.RowIndex]["Depository"] = Depository;
dt.Rows[e.RowIndex]["Mobile"] = Mobile;
dt.Rows[e.RowIndex]["Email"] = Email;
dt.Rows[e.RowIndex]["DPID"] = DPID;
dt.Rows[e.RowIndex]["Client Bef Id"] = Client_Bef_Id;
dt.Rows[e.RowIndex]["UPI ID"] = UPI_ID;
ViewState["dt"] = dt;
Gridview1.EditIndex = -1;
BindGrid();
}

What I have tried:

Input string was not in a correct format.Couldn't store <1561513g> in Pan Column. Expected type is Double.