Redefining alien types on SBCL

If I do this in slime-repl:

(define-alien-type nil (struct foo (str c-string)))

(define-alien-type nil
(struct foo
(a int)
(b (array (* (struct foo)) 100))))

(with-alien ((f (struct foo)))
(slot (deref (slot f 'b) 7) 'a))



and I got the error:

There is no slot named B in #<SB-ALIEN-INTERNALS:ALIEN-RECORD-TYPE
(STRUCT FOO

(STR

C-STRING))>.

This is an inconvenience because I'd like to experiment in the repl.
How can avoid this error?

Note: the same thing happens when redefining generic methods.

.

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