Listbox selects a wrong item with listgroup

Steps to Reproduce

1. load the attached zul
2. check selected item

Current Result

item 1-2

Expected Result

item 1-1

Debug Information

The index of model
0: "group 1"
[1]: "item 1-1"
2: "item 1-2"
3: "group 2"
4: "item 2-1"
5: "item 2-2"

The index of <select> (optgroups are not selectable)
0: "item 1-1"
[1]: "item 1-2"
2: "item 2-1"
3: "item 2-2"

Workaround