[racket] problems with struct datatype

Hi,

I'm trying to use the new struct datatype, but I keep getting the following
error

struct: illegal use of signature form in: (struct posn (x y))

when defining the struct. The old way of defining a struct ((define-struct
...)) works though.

I'm running DrRacket version 5.0.1 on Ubuntu 10.2 that I built from source.

Any ideas on why the error occurs?
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