It's in the changelogs for 5.2: Being the emperor prevents you from being annexed.
Johan said:16th of March
Code:Code: - Implemented a system for attaching units to other nations units in the same province. aka, Coalition Armies for MP. - Fleets not on patrol order will not attempt to intercept enemy fleets when moving and/or carrying troops. - Intercepting enemy fleets that want to avoid it depends on manuever of leaders and light-ship ratio. - Armies on ships trying to disembark will stall for as long as the navy is in combat. - The Sound Toll and the Bosphorus tolls are now more interesting with events and stuff :9 - Non daimyo countries can no longer release daimyo countries. - When negotiating peace with Japan, it's now valid to take the capital of a daimyo if it's their last province. - When sending diplomatic offers as the shogun to countries outside of the shogunate, there's a message telling that you're doing this as Japan. - Some spy activities will now scale in size of the target country instead of being a fixed badboy cost if discovered. [color=red][b]- It is no longer possible to annex the Emperor of HRE.[/b][/color]