Error in SB-IMPL::HASHSET-%INSERT: the value 256 is not of type (UNSIGNED-BYTE 8)

[Paul F. Dietz]

I'm seeing this error when running the rtp tests from ansi-tests. It's difficult to pin it down, since it seems to depend on the state of hash tables. Here's a stack track from a failure:

The value
  256
is not of type
  (UNSIGNED-BYTE 8)
   [Condition of type TYPE-ERROR]

Restarts:
 0: [RETRY] Retry SLIME REPL evaluation request.
 1: [*ABORT] Return to SLIME's top level.
 2: [REMOVE-FD-HANDLER] Remove #<SB-IMPL::HANDLER INPUT on descriptor 4: #<FUNCTION (LABELS SWANK/SBCL::RUN :IN SWANK/BACKEND:ADD-FD-HANDLER) {100A53CBFB}>>
 3: [ABORT] Exit debugger, returning to top level.

Backtrace:
  0: (SB-IMPL::HASHSET-%INSERT #<SB-IMPL::ROBINHOOD-HASHSET SB-KERNEL::MEMBER-TYPE-EQUIV 6082/16384 keys, psl=255 {1000105A63}> #S(SB-IMPL::ROBINHOOD-HASHSET-STORAGE :CELLS #(NIL #<SB-KERNEL:MEMBER-TYPE (M..
  1: ((FLET "WITHOUT-INTERRUPTS-BODY-5" :IN SB-THREAD::CALL-WITH-SYSTEM-MUTEX))
  2: (SB-THREAD::CALL-WITH-SYSTEM-MUTEX #<FUNCTION (FLET SB-THREAD::WITH-SYSTEM-MUTEX-THUNK :IN SB-INT:HASHSET-INSERT-IF-ABSENT) {7FBF75E2DBBB}> #<SB-THREAD:MUTEX taken owner=main thread {10001DBAE3}>)
  3: (SB-INT:HASHSET-INSERT-IF-ABSENT #<SB-IMPL::ROBINHOOD-HASHSET SB-KERNEL::MEMBER-TYPE-EQUIV 6082/16384 keys, psl=255 {1000105A63}> #<SB-KERNEL:MEMBER-TYPE (MEMBER "aAAbABbaBBABaBBbbaB" "䁯ㄼ꧸漍ୢￂ㠲䠅턫豋ᔊ⋝鷅І..
  4: (SB-KERNEL:MAKE-MEMBER-TYPE #S(SB-INT:XSET :LIST-SIZE 3 :DATA ("aAAbABbaBBABaBBbbaB" "䁯ㄼ꧸漍ୢￂ㠲䠅턫豋ᔊ⋝鷅І﻾" "A")) NIL)
  5: (SB-KERNEL::%TYPE-UNION2 #<SB-KERNEL:MEMBER-TYPE (MEMBER "A" "䁯ㄼ꧸漍ୢￂ㠲䠅턫豋ᔊ⋝鷅І﻾")> #<SB-KERNEL:MEMBER-TYPE (MEMBER "aAAbABbaBBABaBBbbaB")>)
  6: (SB-KERNEL::TYPE-UNION2 #<SB-KERNEL:MEMBER-TYPE (MEMBER "A" "䁯ㄼ꧸漍ୢￂ㠲䠅턫豋ᔊ⋝鷅І﻾")> #<SB-KERNEL:MEMBER-TYPE (MEMBER "aAAbABbaBBABaBBbbaB")>)
  7: (SB-KERNEL::SIMPLIFY-UNIONS (#<SB-KERNEL:MEMBER-TYPE (MEMBER "A" "䁯ㄼ꧸漍ୢￂ㠲䠅턫豋ᔊ⋝鷅І﻾")> #<SB-KERNEL:MEMBER-TYPE (MEMBER "aAAbABbaBBABaBBbbaB")>))
  8
[...]

(x86-64, "2.2.11.99-aa65f0355")