Flex - ComboBox has lines around it

I'm stumbling my way through designing my first Flex app, using Flex Builder 4 Beta 2. I'm trying to use a ComboBox, but they always seem to have lines around it and I can't figure out how to get rid of them.

All I did was drag the ComboBox from the Controls section in the Design section of the UI onto the screen. I haven't changed any se开发者_StackOverflowttings, no special css, just used the default ComboBox and this is what I get.

Any ideas?

---

Looks like it was a bug in the beta. Now that the full version is out, we've upgraded to Flash Builder 4 Premium and I no longer see this problem.