datefield and null values

Hi
I have store filled with list of objects, which has nullable datetime property (when record is rendered to client field has null value).

Then I have formpanel with datefield which has dataindex set to this nullabledatetime property.

When I try call updaterecord (with form.getForm().updateRecord(form.record) ) It changes null values to empty string.

Is there any way how to set datefield to work with null values instead of empty strings?

Thanks