ORA-01722: Nombre non valide

  Hi,

Please consider this case :

table : gl_je_lines bal , field : reference_2
table : po_headers_all poh, field : po_header_id

i have the following comparison in a where clause :

to_number(bal.reference_2) = poh.po_header_id (+)

when running the statement, i get sometimes this error : ORA-01722:     Nombre non valide.

what is the best way to do the comparison in order to get rid off this error ?

the types are :
reference_2 : varchar2 (240)
po_header_id  : number

Thanks in advance.
Regards.