date_combo_validate breaks when date field is within a #tree -- stores empty value

How to replicate:

Create a form in which a date_combo element is nested within other form elements with #tree set to true.

What will happen:

The date value will fail to be stored. The field will be empty after saving.

The problem seems to be that date_combo_validate assumes that the field name and delta will be found in #parents[0] and #parents[1], respectively. This assumption won't be valid if the field is within a tree structure.