GHC6.12.2-RC1 runtime error: Invalid object in isRetainer(): 32

Against GHC6.12.2-RC1:

Running Combinatorrent, version g2a3dff0 with retainer profiling +RTS -hr -RTS produces the following bug:

This is Combinatorrent ☠ version v0.1.1-180-g5334c4f
  For help type 'help'

Process.TorrentManager:	Process exiting due to ex: foo.torrent: openBinaryFile: does not exist (No such file or directory)
PeerPool:	Process exiting due to ex: thread blocked indefinitely in an MVar operation
Combinatorrent: internal error: Invalid object in isRetainer(): 32
    (GHC version 6.12.1.20100330 for x86_64_unknown_linux)
    Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
Aborted

I can't select the 6.12.2-RC1 as the version with this problem in the drop-down list. The version of combinatorrent with v0.1.1-180-g5334c4f, publicly available at

http://github.com/jlouis/combinatorrent/commit/5334c4f767970b2bd697ab9e86bdf66258b54de7

can reproduce the problem: Run ./Combinatorrent +RTS -hr -RTS foo.torrent (which doesn't have to exist) to get the above error message.

This is also a regression: Last known version that works with retainer profiling: GHC 6.12.1, if you want to bisect the problem.

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Trac field	Value
Version
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Runtime System
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