I agree with the other answerers who say that this looks like a bug. Specifically it appears to be a bug in C# runtime binding layer, though I have not investigated it thoroughly.
I apologize for the error. I’ll report it to the C# 5 testing team and we’ll see if it has already been reported and fixed in C# 5. (It reproduces in the recent beta release, so it is unlikely that it has already been reported and fixed.) If not, a fix is unlikely to make it into the final release. In that case we’ll consider it for a possible servicing release.
Thanks for bringing this to our attention. If you feel like entering a Connect issue to track it, feel free to do so and please include a link to this StackOverflow question. If you don’t, no problem; the test team will know about it either way.