SQL & PL/SQL

Hi I have data as below in table. the column data type is varchar2. I am getting "ORA-01858: a non-numeric character was found where a numeric was expected" error if i try to get output for any month but without the error clause the query works fine.

char_value

7/11/1973

4/15/1974

1/20/1975

3/10/1975

6/8/1976

9/20/1976

11/2/1976

2/28/1977

below query not working

select * from 
(select to_date(char_value, 'mm/dd/yyyy') service_date,
to_char(to_date(char_value, 'mm/dd/yyyy') , 'mm') as service_month
from user_char)
where service_month = '07'

below query works fine.

select * from 
(select to_date(char_value, 'mm/dd/yyyy') service_date,
to_char(to_date(char_value, 'mm/dd/yyyy') , 'mm') as service_month
from user_char)