Surprise printing to string (common lisp)

Following the documentation for WITH-OUTPUT-TO-STRING and GET-OUTPUT-STREAM-STRING, I expect the following to work, and they do:

(print
 (with-output-to-string (sb nil)
   (format sb "~A " "hello, ")
   (format sb "~A~&" "world")
   sb))

(print
 (let ((sb (make-string-output-stream)))
   (format sb "~A " "hello, ")
   (format sb "~A~&" "world")
   (get-output-stream-string sb)))

However, the following, which is close to one of the examples in WITH-OUTPUT-TO-STRING, does not:

(print
 (with-output-to-string (sb (make-array
                             '(0)
                             :element-type 'base-char
                             :fill-pointer 0
                             :adjustable t))
   (format sb "~A " "hello, ")
   (format sb "~A~&" "world")
   sb))

producing, instead, the output stream, itself, rather than the string held within:

#<SB-IMPL::FILL-POINTER-OUTPUT-STREAM {1005FBE523}>

I have not been able to find a way to extract the string inside the output stream. I suspect that it has something to do with dynamic extent, but my understanding falters, here.

Obviously, I have kosher ways of achieving the desired result, so I am merely curious to discover my misunderstanding of the language.

Because the documentation says that the results are undefined for GET-OUTPUT-STREAM-STRING on a stream not created by MAKE-STRING-OUTPUT-STREAM, I am not surprised that the following doesn't work:

(print
 (with-output-to-string (sb (make-array
                             '(0)
                             :element-type 'base-char
                             :fill-pointer 0
                             :adjustable t))
   (format sb "~A " "hello, ")
   (format sb "~A~&" "world")
   (get-output-stream-string sb)))

but I'd still be grateful for finding a way of extracting the string in my third example.