Error With MAX Function

With this statement I get an error where "Column QuoteHeaderID does not belong to table. " because Object reference isn't being set to an instance of an object, yet if I remove the max part it functions correctly.



Code Snippet
string num1 = "";
string sqlString = "SELECT MAX(QuoteHeaderID) FROM QuoteHeader";
try
{
cmd = new OleDbCommand(sqlString, connectCmd);
cmd.Connection.Open();
cmd.ExecuteNonQuery();
oleDbDataAdapter = new OleDbDataAdapter(cmd);
oleDbDataAdapter.Fill(dt);
if (dt.Rows.Count > 0)
{
num1 = dt.Rows[0]["QuoteHeaderID"].ToString();
}
}
catch(Exception ex)
{
string s = ex.Message;
DisposeResources(ref oleDbDataAdapter, ref ds, ref connectFill, ref connectCmd, ref cmd);
cmd.Connection.Close();
return 0;
}