Saving HTML in Editor gives 'potentially dangerous Request.Form value'

I'm trying to save HTML in an Editor field. Only admins have access to this page, so there's no XSS issue. I get an ASP.NET error - 'A potentially dangerous Request.Form value was detected from the client'. I tried what was suggested at https://datatables.net//forums/discussion/comment/98528/, in both the question and answer, but it didn't help.