ORA-06502 PLSQL numeric or value error character to number conversion error ORA-06512 at LDBO.SP_SERSEQ

SELECT voucher FROM LEDGER WHERE FIRMNUMBER='APXC-00001' and voucher like '%JV%' and Regexp_Like((SUBSTR(VOUCHER,-7,7)), '[[:alpha:]]');

SELECT MAX(SUBSTR(VOUCHER,-7,7)) FROM LEDGER WHERE FIRMNUMBER='APXC-00001' and voucher like '%JV%' and Regexp_Like((SUBSTR(VOUCHER,-7,7)), '[[:alpha:]]');
result should be numeric