Torbutton's "Test Settings" fails with "No TorCheckResult id found (response not valid XHTML)"

When you go to Torbutton Preferences and click "Test Settings", it complains that No TorCheckResult id found.

What's going on inside Torbutton to make this happen?

Confirmed in both TBB 3.0b1 (by me) and 2.4.18-rc-1 (by Erinn).

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