Umbraco's TinyMCE error: 'Cannot read property parentsRequired of undefined'

If you find that the “Formats” list in an Umbraco RTE refuses to open, and the Javascript console contains this error:

"Cannot read property ‘parentsRequired’ of undefined"

Check the stylesheet that the RTE is using.  In my case, adding this style had triggered the issue:


Although there’s nothing wrong with this style itself, the problem is in what TinyMCE considers to be “valid” HTML elements.  Why is “small” not valid?  No idea!  But you can fix this by opening /config/tinyMceConfig.cfg, finding the <validElements> section and appending this:


And that’s it!  You can probably substitute “[class]” with other attributes if you need to, or even just [*].  I’m not really sure what all the implications of that are (let me know if you find out), so I just went with [class] like some of the other properties have.

Credit to Eric Schrepel for helping me figure this one out at