Button updating a RadAjaxPanel

I encountered this error today when putting a RadAjaxPanel around a load of form controls and a button and linking them to a RadAjaxManager and RadAjaxLoadingPanel:

Invalid postback or callback argument. Event validation is enabled using in configuration or <%@ Page EnableEventValidation="true" %> in a page.

The solution it seems is to put EnableEventValidation="false" in the Page directive at the top of the aspx page.