ADO Insert causes Invalid Variant Type Conversion error

I have a simple master-detail tables structure with ADO: ORDER and
ITEMS.
I can browse through the data without a problem, but when I issue the
command "ORDER.Insert", I get the error message: "Invalid Variant Type
Conversion".

What am I doing wrong here??? The detail table (ITEMS) have the
following properties:

MasterSource: ORDER
MasterField: ID
IndexFieldName: ITEM_ID

I read about Variant Type to NULL conversion problems, and maybe it's
got something to do with primary and foreign keys being null on
insert, but how can I make it work???

TIA for your urgent replies!!!

Jessica