ClientDataset Delta Empty if Clear TField?

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Hi everybody!
I am using Delphi-7, Midas, TClientDataset...
May be somebody knows how i can resolve the following problem: if I
do the following modification to some record: Clear Field from some
value to NULL - and then check Delta records for my Dataset - there
are NO records concerning cleared Field.Respective Delta record with
usModified status exists but IS EMPTY.
Does somebody knows how to resolve problem with parsing Delta in this
case?
(By the way - I parse Delta-records for create SQL statements for
modification of DB records). And it's works perfect for any cases but
described above...