Parse error when using an expression as #class

[progval]

When using something that is not a string a the #class attribute of an object, I get a parse error.
I would understand if it was a runtime error, but why does the parser error? According to ljs/ljs_parser.mly, #class expects an expression (exactly as for #proto, but in the case of #proto, an expression works)

vlorentz@gommier:~/js/LambdaS5/tests(working)$ echo "{[#proto: 5]}"  | ../obj/s5.d.byte -load init.heap /dev/stdin -continue-s5-eval
object
vlorentz@gommier:~/js/LambdaS5/tests(working)$ echo "{[#class: 'foo']}"  | ../obj/s5.d.byte -load init.heap /dev/stdin -continue-s5-eval
object
vlorentz@gommier:~/js/LambdaS5/tests(working)$ echo "{[#class: 5]}"  | ../obj/s5.d.byte -load init.heap /dev/stdin -continue-s5-eval
Fatal error: exception Failure("parse error at /dev/stdin:1:11-11; unexpected token 5")