AJAX :: ModalPopupExtender Show Condition Not Working As Expected

I use ModalPopupExtender in my page

I have 2 imagebutton

1-Imgcab
2-imgenter

Below are my imagebuttons event code

protected void Imgcap_Click(object sender, ImageClickEventArgs e) {
string data = Server.UrlEncode(Txtbeh.Text);
SqlCommand _cmd = new SqlCommand("housepass", _cn);
_cmd.CommandType = CommandType.StoredProcedure;
_cn.Open();

[Code]....

when I click on ImgEnter it show popup menu ModalPopupExtender.Show();

It is correct problem is that when I click on imgcap if it run first ELSE condition it does Session["MessageError"] = true; and it show popupmenu (it show ModalPopupExtender)

I want it just show popup menu (ModalPopupExtender.) just when I click on imgEnter button ....