I'm getting the same key not found error as this post when using AncestorOrDefault. In my case the doc type I'm looking for definitely exists. Works fine in my development environment, but not on test or production servers. My work around for now is to manually crawl up the tree by grabbing the id of each parent one at a time. So instead of 

var events = (from ev in ctx.Events
where ev.AncestorOrDefault<SiteRoot>().Id==siteId...

I'm doing

int parentId = ctx.EventsFolder.Where(p => p.Parent<SiteRoot>().Id == siteID).First().Id
var events = (from ev in ctx.Events
where ev.Parent<EventsFolder>().Id==parentId...

Thought it might have something to do with generating the context from my development environment, but when I replace with code generated from the production environment, it still throws the error. And still works in dev.

This is 4.5.2 for .net 3.5 running on IIS 5?(dev - XP), 6? (test - Server 2003), 7.5 (prod - server 2008). All the dlls were copied from the same source. Any ideas?

Thanks
Jeff