Oracle : SQLCODE and SQLERRM in Inserts

Today I came across an unexpected problem when executing an exception block inside a procedure :

My code was something like this :

insert into dc_errors (error_message,error_date) values (SQLERRM, SYSDATE);

and I allways obtain an annoying PL/SQL: ORA-00984: column not allowed here.

To solve this problem I've changed my code into :

v_exception := substr(SQLERRM, 1, 256);
insert into dc_errors (error_message,error_date) values (v_exception, SYSDATE);

with v_exception as varchar2(256) .