dr=cmd.ExecuteScalar(); it is conduct the error and return a conduct the error please check and solve this error sir please

public int membercount(string tbl, string condition)
{
  string query = null;
  if (condition.Trim().Length > 0) 
  {
    query = query + "where" + condition;
  }
  else
  {
    query = query;
  }
  cmd = new SqlCommand(query, con);
  dr = cmd.ExecuteScalar();
  return a;
}