Why can't I share the clipboard using TightVNC server on Windows from Mac's screen sharing?

I have the TightVNC server running on Windows 7 Home Edition and I'm using Screen Sharing on Mac OS X. On the Mac, the clipboard sharing button is greyed out:

If I copy and paste in either OS, the clipboard isn't transmitted. There appear to be no options in the TightVNC server configuration.

Any ideas how to get clipboard sharing to work?

Edit: Clipboard sharing works fine if I use the TightVNC viewer from a Windows machine. However, there's no TightVNC client for Mac OS X.