Using Cached Update, the last Data is not deleted

Hi. Using Cached Update, the last Data is not deleted.
I am working with Delphi3(C/S) and Oracle7.1.
To Manupulate Date, I use Query, UpdateSQL Component and DBGrid Control.
Through Cached Update Method of Query Component, Oracle's Remote Data
is well controlled(Insert, Delete, Update). But I have one serious
problem I can't solve it. When Query's Recordcount is
more than 1, the following Code is good work. But when Query's record
count is one, Delete Operation is not work, error raise.
The Error Text is following:

"Field Value Required.
ORA-01400. mandatory (NOT NULL) Column is missing or NULL during
insert"

Though I operated Delete Command, error is raised 'during insert'.

I think db.pas has some bug.
I expect your sincerely advice or solution.

(Status)
Coding:

procedure frmMain.Query1fterPost(DataSet: TDataSet);
begin
    DataBase1.ApplyUpdates([Query]);  //To commit Data on database.
end;

With DBGrid Component, Query, UpdateSQL.
In Query's propery,
  CachedUpdates := true;
  UpdateObject := UpdateSQL1;