Cannot use custom visuals in Power BI Desktop

Custom visualizations work in web preview, but not in Power BI Desktop. I can import the .pbiviz file and drop the custom visual into a report, but as soon as you try to drop any data into it, you get a message that says:

Your version of Power BI Desktop does not support this custom visual. Please update Power BI Desktop to the latest version.

It does this with every custom visual I have tried so far, including the stock, unmodified template (pbiviz new test / pbiviz package) and the sample bar chart from github.

I've tried uninstalling and reinstalling freshly-downloaded PBI Desktop - no help.