ORA-06502: PL/SQL: numeric or value error: character to number conversion error

L_a_u_r_a

I get this error sometime in a report in APEX. The application passes 2 parameters with a date data type,  to_date(:P1_FROM_DATE, 'DD-MON-YYYY' ) and  to_date(:P1_TO_DATE, 'DD-MON-YYYY' ). The Page items are Date Pickers with a format mask DD-MON-YYYY. They are passed into a package that creates a table. I then query the table in APEX to get the report. The error doesn't seem consistent which is making it difficult to find the cause. If I pick dates in December, I get results. If I pick dates in June, I get the error: ORA-06502: PL/SQL: numeric or value error: character to number conversion error.