Why getting null instead of code

CREATE OR REPLACE FUNCTION fngetlastcodeofvillage(
    text,
    text,
    text)
  RETURNS text AS
$BODY$
    DECLARE 
   
    schoolcode text;
      BEGIN
      SELECT schcd into schoolcode FROM villagerural_school_form  where districtna=$1 and talukname=$2 and villagenam=$3 order by id desc limit 1;
      IF schoolcode IS NULL THEN
        return (select census2011||'01' from villagerural b where b.districtna=$1 and b.talukname=$2 and b.villagenam=$3);
       else
        return (cast(cast(schoolcode as bigint)+1 as text ));
     END IF;
end
 $BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;
ALTER FUNCTION fngetlastcodeofvillage(text, text, text)
  OWNER TO postgres;

public string Getschoolcodebyddl(String districtna, String talukname, String villagename)
        {

            return Helper.ExecuteSelectCommand("select * from fngetlastcodeofvillage('" + districtna + "','" + talukname + "','" + villagename + "');", CommandType.Text).ToString();
        }