Query problem

[Security]

UPDATE HeaderSa SET ObsLitige = (SELECT [Client].[Obs] from HeaderSa
,client
WHERE [HeaderSa].[CodeClient]=[Client].[CodeClient] And
[HeaderSa].[TypeLitige] Like '*Client Bloqué*');

gives me the error message : 'memo field incorrect in the request'

the Obs ... fields are of memo type, what is the problem ??

thanks