Asp.net中的where子句在下面的代码中返回null异常

 VotingDemoDataContext db = new VotingDemoDataContext();
        var x = (from name in db.Elections select name);
        int n = x.Count();
        Election election = new Election();
        election.ElectionID = n.ToString();
        Constituency c=new Constituency();
        election.ConstituencyId = Convert.ToInt32(from m in db.Elections where (c.ConstituencyName==ddlConstitency.SelectedItem.Text) select m.ConstituencyId);
        election.Date = Convert.ToDateTime(txtDate.Text);
        election.ElectionType = ddlEType.SelectedItem.Text;
        election.EO = txtEOName.Text;
        election.EOPassword = txtEOPassword.Text;
        db.Elections.InsertOnSubmit(election);
        db.SubmitChanges();
        txtEOPassword.Text = "";
        txtDate.Text = "";
        txtResultDate.Text = "";
        ddlConstitency.SelectedIndex = 0;
        ddlEType.SelectedIndex = 0;
        txtEOName.Text = "";

此代码为以下行返回null异常：election.ConstituencyId = Convert.ToInt32(from m in db.Elections where (c.ConstituencyName==ddlConstitency.SelectedItem.Text) select m.ConstituencyId);