ORACLE - SELECT which returns only 1 row

This example returns only one (or no) row:

declare
  iPrice number;
begin
  ..
  select kost_soll_voll_mz into iPrice
  from erhtks
  where 
  tl_nr = pTl_nr and 
  tl_fam = pTl_fam and 
  tl_atn = pTl_atn and 
  koa_nr = pKoa_nr and 
  kal_auf_nr_ursp = pKal_auf_nr and 
  rownum = 1;