SQL & PL/SQL

I have a procedure where I am selecting a clob locator into a clob var and then trying to update the clob. I have tried copy and writeappend but I keep getting ORA-22275: invalid LOB locator specified. Anyone have any ideas?

Here's the code:
I'm passing in a clob variable as_email_body

lbl_clob clob;

SELECT email_body
INTO lbl_clob
FROM email_info
WHERE id = 1;

dbms_lob.WriteAppend ( lbl_clob, dbms_lob.GetLength(as_email_body), as_email_body);

When the writeappend failed I thought maybe I was using the wrong function so I tried copy
dbms_lob.Copy ( lbl_clob, as_email_body, dbms_lob.GetLength(as_email_body), dbms_lob.GetLength(lbl_clob)+ 1, 1);

Thanks!