Append to a multi-lined element?

[Zinnecker, Albin, AZV]

Hi all,
 
I'm on a 4.6B release and tried to create a container-operation that appends
a single-lined element to a multi-lined element. Both elements refer to the
same structure (the elements are defined with a tablename but no fieldname -
so far so good!). The error that's raised is some kind of strange because it
says: "operation APPEND between data type 'u' and data type 'u' not allowed"
(No. WL 490).
 
The two elements are absolute identical except that one is single-lined and
the other multi-lined! By the way, it doesn't even work with two exact equal
elements!
 
Has anyone done such an assignment before? I think it's a standard
function!?
 
Many thanks in advance,
 
Regards,
 
Albin Zinnecker
a.zinnecker at gewkoelnag.de