To_Number Conversion

Hi

I'm working on Oracle/PLSQL using Sql developer. I'm have a trouble trying to convert to numeric using TO_NUMBER.

When i run the query:

SELECT TO_NUMBER(column1) FROM Table1

it works fine

But when i run below query:

SELECT * FROM Table1 t1 

LEFT JOIN Table2 t2 ON TO_NUMBER(t1.REF_NO) = TO_NUMBER(t2.REF_NO)

it throws error : ORA-01722: invalid number

NB: REF_NO column is of datatype CHAR for both tables.

Thanks in advance.

L.M