SQL & PL/SQL :: Function That Accepts 2 Dates - Not A Valid Month Error

I have function that accepts 2 dates namely sDate and eDate i.e start and end dates. See the declaration part of the function:

startDate eab.eod_date%type;
endDate eab.eod_date%type;

begin
startDate := to_date(sDate,'DD-MON-YY');
endDate := to_date(eDate,'DD-MON-YY');
end

When I called the function it gives me the following error: ORA-01843: not a valid month..And I tried to simulate the process, the below code also show the same error:

select TO_DATE(to_date('13-DEC-09', 'dd-MON-yy', 'nls_date_language=english'),'DD-moN-YY') from dual;

how I can check that error. I need to pass dates to my function.