Sanity bug found by +RTS -DS in Agda

With my HEAD ghc, I built Agda with -debug, and get, reproducibly (on an Agda call that normally segfaults after some 12 minutes):

time agda +RTS -DS -C0 -i0  -K64M -M12G -H12G -S -RTS -i . -i /var/tmp/AGDA/lib/src Categoric/OrderedMonad/Kleisli/Comp/AssocProofterm.lagda
agda -i . -i /var/tmp/AGDA/lib/src Categoric/OrderedMonad/Kleisli/Comp/AssocProofterm.lagda +RTS -DS -C0 -i0 -K64M -M12G -H12G -S 
    Alloc    Copied     Live    GC    GC     TOT     TOT  Page Flts
    bytes     bytes     bytes  user  elap    user    elap
   570504     67280     87816  2.33  4.74    2.33    4.75    0    0  (Gen:  1)
  8676120    935840   1054664  2.86  3.63    5.24    8.43    0    0  (Gen:  0)
  9560288   2270144   2423920  2.91  2.93    8.17   11.38    0    0  (Gen:  0)
 11102944   2983776   4004992  2.92  2.95   11.13   14.35    0    0  (Gen:  0)
agda: internal error: ASSERTION FAILED: file rts/sm/Sanity.c, line 664

    (GHC version 6.13.20100815 for x86_64_unknown_linux)
    Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
Aborted

real	0m17.637s
user	0m14.026s
sys	0m3.519s

What should I look into for this?

Trac metadata

Trac field	Value
Version	6.13
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown/Multiple
Architecture	Unknown/Multiple