[Info-ingres] correlated update with inline view?

[nikosv]

The following query works fine and does return not-null results :

select * from   akana_eisa_epexa ak where   
     c_grnoe = (
     select c_grnoe from (select * from temp2,(  
                         select am_asqenh, d_ekdoshs,  max(d_episkep) as
     d_episkep
                         from temp2     
                         group by am_asqenh, d_ekdoshs) a
                         where temp2.am_asqenh=a.am_asqenh and   
                         temp2.d_ekdoshs=a.d_ekdoshs and
                         temp2.d_episkep=a.d_episkep) b
                                             where ak.am_asqenh=b.am_asqenh     
                                             and ak.d_ekdoshs = b.d_ekdoshs
                                             )
but the following update dies with error 
E_AD1012 An attempt to place a null value in a non-nullable datatype.
    (Fri Jan 15 09:46:30 2016) 

update akana_eisa_epexa ak set   
     c_grnoe = (
     select c_grnoe from (select * from temp2,(  
                         select am_asqenh, d_ekdoshs,  max(d_episkep) as
     d_episkep
                         from temp2     
                         group by am_asqenh, d_ekdoshs) a
                         where temp2.am_asqenh=a.am_asqenh and   
                         temp2.d_ekdoshs=a.d_ekdoshs and
                         temp2.d_episkep=a.d_episkep) b
                                             where ak.am_asqenh=b.am_asqenh     
                                                and ak.d_ekdoshs = b.d_ekdoshs
                                             )

or even tried with the FROM syntax:

update   akana_eisa_epexa ak1
   from akana_eisa_epexa ak  											 
     c_grnoe = (
     select c_grnoe from (select * from temp2,(  
                         select am_asqenh, d_ekdoshs,  max(d_episkep) as
     d_episkep
                         from temp2     
                         group by am_asqenh, d_ekdoshs) a
                         where temp2.am_asqenh=a.am_asqenh and   
                         temp2.d_ekdoshs=a.d_ekdoshs and
                         temp2.d_episkep=a.d_episkep) b
                                             where ak.am_asqenh=b.am_asqenh     
                                                and ak.d_ekdoshs = b.d_ekdoshs
                                             )
where ak1.am_asqenh=ak.am_asqenh


still the same error

is something like that even possible?