TDBEdit with masks - how to make it work?

Hi people,

I have a TDBEdit used to enter an ID.  I wish to prevent the user including
blanks or non-alphanumerics  in the ID. I thought that an edit mask would be
the right idea but can't get it working
.
I derived a component 'TMyDBMaskedEdit'  from TDBEdit to gain access to the
protected EditMask property of TCustomerMaskEdit, with the method:

procedure TMyDBMaskedEdit.SetEditMask(myMask: string);
begin
  EditMask := myMask;
  MessageDlg('IsMasked = ' + IntToStr(Ord(IsMasked)), mtInformation, [mbOK],
0);
end;

I set the mask in the FormCreate event  of the form containing my
TMyDBMaskedEdit  (named dedRtuId) :

procedure TRtuCfgForm.FormCreate(Sender: TObject);
begin
     DataModule1.dedRtuId.SetEditMask('LAAAAAAAAAAAAAAA;0;*');
end;

So I'm trying to restrict user input to a letter followed by up to 15
alphanumerics.
It compiles and runs but doesn't restrict input at all.
The MessageDlg call shows that IsMasked is set to True.

Any ideas on how to get this working?

Cheers,
Craig.