stack overflow with tfield's mask

I use a dbgrid, connected to a clientdataset with a field list in it. In the
field editor I set the editmask for a particular date field to:
"!99/99/0000;1;_" and when the user tries changing the field in the grid I
get a 'stack overflow' exception.
Removing the mask (and doing a validation myself) solved the problem.
I don't use any of fields event, and I only use the afterpost and
onnewrecord events of the dataset.
I use Delphi 6 on Win98SE.
Any idea of the reason?
Thank you in advance