[plt-scheme] Self evaluating "objects"

Hello!

I've tried the following expression in PLT 4.0.2 and Gambit-C 4.2.8 and  
got different answers:

(eval (lambda (x) x))


PLT
> (eval (lambda (x) x))
#<procedure>


Gambit-C
> (eval (lambda (x) x))
*** ERROR -- Ill-formed expression
#<procedure #4>
1>

I've browsed trough R5RS and R6RS and I was not able to figure out which  
behavior is the right one.
Is this implementation dependent or it is prescribed by some standard ?