DataRow find problem C#

public static DataSet dbValueGet() {
            DataSet ds = new DataSet();
            string connection = ConfigurationManager.ConnectionStrings["LocalConnection"].ConnectionString;
            SqlConnection con = new SqlConnection(connection);
            string sql = "SELECT * FROM Tags; SELECT * FROM TagLanguage";
 
            //Query tables and add to Dataset.
            SqlCommand cmd = new SqlCommand(sql, con);
            SqlDataAdapter daTagLanguage = new SqlDataAdapter(cmd);
            daTagLanguage.Fill(ds);  
            return ds;
        }
 
 
THIS IS THE METHOD THAT FAILES
public static string cacheValueGet(string tagname, out bool isDefined) {
            string rtnStr = "Undefined";
            isDefined = false;
 
            DataSet ds = null; 
            if (ds == null) {
                ds = db.dbValueGet();
                //table index 1 = [dbo.Tags]
                //table index 0 = [dbo.TagLanguage]
 
                //Set tags Primary Key
                DataTable dtTags = ds.Tables[0];
                DataColumn[] dcTagsPrimary = { dtTags.Columns["id"] };
                dtTags.PrimaryKey = dcTagsPrimary;
 
                //Set tags Primary Key
                DataTable dtTagLangs = ds.Tables[1];
                DataColumn[] dcTagsLangsPrimary = { dtTagLangs.Columns["TagID"], dtTagLangs.Columns["LangID"] };
                dtTagLangs.PrimaryKey = dcTagsLangsPrimary;
 
                //Set up the dataset Relationship
                DataColumn[] dcTagsForeign = new DataColumn[] { ds.Tables[0].Columns["id"] };
                DataColumn[] dcTagsLangsForeign = new DataColumn[] { ds.Tables[1].Columns["TagID"] };
 
                DataRelation TagsLangs_Tags = new DataRelation("TagLangRelationship", dcTagsForeign, dcTagsLangsForeign);
                ds.Relations.Add(TagsLangs_Tags);
 
                //Add the dataset into cache
 
            }
                      DataRow drFiltered = ds.Tables[0].Rows.Find(tagname);
       
 
                   if (drFiltered != null) {
                //then you know you have the records filtered.
            
                return drFiltered[0].ToString();
 
            }
            else {
                isDefined = false;
            }
 
            return rtnStr;
}