SQL & PL/SQL

I am trying to execute the following dml but i am gettig the abve mentioned error

Is it possible to recitfy this error without creating a primary key on the column c1 of TABLE a

UPDATE   (
SELECT   a.c1
            FROM    a,  b
           WHERE       a.c1 IS NULL
                   AND a.c2 = b.c2
                   AND a.c3 = b.c3
                   ) c
   SET   c.c1 = 'drop';