Oracle DB - Select liefert Fehler aufgrund ungültigen Monats obwohl die betreffenden Zeilen ausgeschlossen sind

create or replace
function IsDate (str varchar2) return varchar2 is
inDate varchar2(40);

FUNCTION dateCheck (inputDate varchar2, inputMask varchar2)
RETURN varchar2
IS
dateVar date;

BEGIN
dateVar:= to_date(inputDate,inputMask);
return 'true';  
exception
when others then
return 'false';  
END;

BEGIN
inDate:= trim(str);
if dateCheck(inDate, 'yyyymmdd') = 'false'  
then
return 'false';  
else
return 'true';  
end if;
END;

Select wadat_ist, isdate(wadat_ist) from zsd_paletten_view;

Select to_date('wadat_ist', 'YYYYMMDD'), isdate(wadat_ist) from zsd_paletten_view where isdate(wadat_ist) = 'true';  

Error report:
SQL Error: ORA-01841: (Volles) Jahr muss zwischen -4713 und +9999 liegen und darf nicht 0 sein
01841. 00000 -  "(full) year must be between -4713 and +9999, and not be 0"  
*Cause:    Illegal year entered
*Action:   Input year in the specified range

CREATE OR REPLACE FORCE VIEW "PALETTEN"."ZSD_PALETTEN_VIEW" ("MANDT", "KUNAG", "KUNNR", "ZZPAL_ART", "WADAT_IST", "VBELN", "VTEXT", "ZZLIEFG", "ZZRUECKG", "NAME1", "NAME2") AS   
  SELECT "MANDT",  
          "KUNAG",  
          "KUNNR",  
          "ZZPAL_ART",  
        "WADAT_IST",  
          "VBELN",  
          "VTEXT",  
          "ZZLIEFG",  
          "ZZRUECKG",  
          "NAME1",  
          "NAME2"  
     FROM zsd_paletten@sap_prod
     WHERE isdate(wadat_ist) = 'true';  

WHERE isdate(wadat_ist) = 'true';