CHICKEN 6 u8vector vs bytevector type

[Diego A. Mundo via Chicken-users]

I was attempting to compile the following:

  (import srfi-4)

  (: a u8vector)
  (define a (make-u8vector 4 0))

And saw:

  Warning: Invalid assignment
    In file `/tmp/u8.scm:6',
    At the toplevel,
    In assignment:

      (set! a (chicken.number-vector#make-u8vector 4 0))

    Variable `a' is assigned invalid value.

    The assigned value has this type:

      bytevector

    The declared type of `a' is:

      (struct u8vector)

It works if I declare it a bytevector. Is this a bug? All the srfi-4 u8 
procedures are declared to operate on bytevector, which appears to be a 
distinct type from u8vector.

Diego