Ora-1722 - Invalid number

Hi, Folks.

     I have a table called s_BENNER.TEMP_FURNASIMPORTABENEFICIARIO with the following structure:

Nome                        Nulo     Tipo         
--------------------------- -------- -------------
HANDLE                      NOT NULL NUMBER(10)   
Z_GRUPO                              NUMBER(10)   
C1C1TIPOREGISTRO                     VARCHAR2(1)  
C1694C1696_CONVENIO                  VARCHAR2(3)  
NOMEARQUIVO                          VARCHAR2(100)
C186C187                             VARCHAR2(2)  

  When I run the query below:

SELECT HANDLE,C186C187,TO_NUMBER(C186C187)
        FROM S_BENNER.TEMP_FURNASIMPORTABENEFICIARIO
        WHERE NOMEARQUIVO LIKE '\\10.1.0.154\TRANSF_BENNER\BENEFICIARIOS\IMPORTACAO\1B270612.TXT'
                    AND SUBSTR(C13C21,3,7)='0158185';

It returns:

    HANDLE C186C187 TO_NUMBER(C186C187)
---------- -------- -------------------
    188266 00                         0
    188267 01                         1

But , when I want to return only 'C186C187' = 0  as Number, it occurs an error ORA-1722 as below