code-char doesn't return nil on failure

The HyperSpec entry for code-char [1] reads:

"Returns a character with the code attribute given by code. If no such character exists and one cannot be created, nil is returned."

On the following code, however, a STREAM-ENCODING-ERROR is signaled instead of nil being returned:

This is SBCL 0.9.8, an implementation of ANSI Common Lisp.
More information about SBCL is available at <http://www.sbcl.org/>.

SBCL is free software, provided as is, with absolutely no warranty.
It is mostly in the public domain; some portions are provided under
BSD-style licenses. See the CREDITS and COPYING files in the
distribution for more information.
* (code-char (1- char-code-limit))

debugger invoked on a SB-INT:STREAM-ENCODING-ERROR in thread
#<THREAD "initial thread" {A7BE341}>:
  encoding error on stream #<SB-SYS:FD-STREAM for "standard output" {A7BE591}>
  (:EXTERNAL-FORMAT :ASCII):
    the character with code 1114111 cannot be encoded.

Type HELP for debugger help, or (SB-EXT:QUIT) to exit from SBCL.

restarts (invokable by number or by possibly-abbreviated name):
  0: [OUTPUT-NOTHING] Skip output of this character.
  1: [ABORT ] Exit debugger, returning to top level.

(SB-INT:STREAM-ENCODING-ERROR
 #<SB-SYS:FD-STREAM for "standard output" {A7BE591}>
 1114111)
0]

This bug causes FiveAM [2] to break SLIME [3] when using its gen-character and gen-string generators.

[1] http://www.lisp.org/HyperSpec/Body/fun_code-char.html#code-char
[2] http://common-lisp.net/project/bese/FiveAM.html
[3] http://common-lisp.net/project/slime/