Rails - Jul 2006 - error_message_on / error_messages_for

Hi,

just a simple question between error_message_on / error_messages_for ...
Well, if you use error_messages_for in your web page, there is no 
problem even if anObject==nil (for example , the first time you load 
your form) whereas error_message_on must be use with an instanciated 
object ...

So, you always has to double check if anObject is not nil before using 
error_message_on ?

I don''t understand why there is a difference, I mean why you cannot 
simply use <% error_message_on anObject, aMethod %> without evaluating 
anObject ?

thanks

arnaud