error when dosenot select data from DDL

[ROC]

I had this message \[code\]"Input string was not in a correct format"\[/code\] wheninsert values to DB, when I checked I have DDL but I didnot selectvalue from it so this message appeared , although I make this field inDB allow NULL value.\[code\]protected void BT_submit_Click(object sender, ImageClickEventArgs e){ string File = "~/CvFiles/" + FU_CV.FileName; if (FU_CV.FileBytes.Length > 4194304) { modalpopup.Show(); } else { app.AddApplicant(txt_Mname.Text, Convert.ToInt32(DDL_Dept.SelectedValue)); }}private void loadDepts(){ DDL_Dept.DataSource = d.GetAll(); DDL_Dept.Items.Clear(); DDL_Dept.AppendDataBoundItems = true; DDL_Dept.Items.Insert(0, new ListItem("-All-", "NULL")); DDL_Dept.DataValueField = "id"; DDL_Dept.DataTextField = "name"; DDL_Dept.DataBind();}\[/code\]