DataSet Fill Using Cmd.Parameters - Legal?

A - Must Declare variable @ZipEntry - error generates when I run the code below.  If I hard code the "WHERE ZipCode = 72364" it works (display messed up but it works - don't know how to code in the line breaks yet).  But when I try to pass in the value of a TextBox called txtZipEntry on the page I get this error.  Am I not delcaring @ZipCode in the correct spot ?

Thank you,

public partial class ContractorResultsDisplay : System.Web.UI.Page
{
string ZipEntry;
protected void Page_Load(object sender, EventArgs e)
{

ZipEntry = txtZipEntry.Text;

SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["localhomeexpoConnectionString2"].ConnectionString);
string sql = "Select [City], [StateCode], [AreaCode], [Latitude], [Longitude], [ZipCode] From [ZipCensus11202006] Where ZipCode > @ZipEntry";
SqlCommand cmd = new SqlCommand(sql, con);
cmd.Parameters.AddWithValue("@ZipEntry", ZipEntry);
SqlDataAdapter da = new SqlDataAdapter(sql, con);

DataSet ds = new DataSet();
da.Fill(ds, "ZipCodeRefs");
StringBuilder htmlStr = new StringBuilder("");

foreach (DataRow dr in ds.Tables["ZipCodeRefs"].Rows)
{
htmlStr.Append(dr["City"].ToString());
}

ZipContent.Text = htmlStr.ToString();

}

}