IE sucks again! Without a decent debugger (like Firebug), I have to read prototype.js (version: 1.4.0_pre11) and spend hours trying to pinpoint the fault. This alien error appears to be this block: if (receiver) { if (this.options.insertion) { new this.options.insertion(receiver, response); } else { receiver.innerHTML = response; } } Particularly this line: receiver.innerHTML = response; I am trying to rewrite a tbody of a table but IE does not like that. This link is insightful (RobG): ...don't use innerHTML to modify tbody, tr, th or td elements, use DOM (they actually refer to a 'table object model', but DOM is a better bet). If you are going to use innerHTML near a table, write the entire table or just cell content, no in between.