not null filter

I have a simple sql, looking at the predicate information from the dbms_xplan

I have on the first position the top position filter( IS NULL).

I get the is null part but uncertain why I do not see a column reference,

I expect to see a column there .

Is this an error from dbms_xplan or what does this tell me ?

Appreciate if anyone can explain it to me...

SELECT A.row_id       FROM   siebel.s_org_ext A,

siebel.s_org_ext_xm C       WHERE  A.x_pr_comp_code_argus NOT IN

(SELECT row_id                                             FROM

siebel.s_org_ext_xm COM1