I would be surprised if this is happening by design, otherwise that's a really annoying design flaw. I was able to do it on both of my computers with Windows 7 v7000 without any problems.
I don't know if it's relevant, but if I run Windows Defender manually, it loads up and says, "A problem caused this program's service to stop. To start the service, click the Start now button or restart your computer."
If I click "Start now," it will sit for 5 minutes and then give some sort of time-out error.