program exits at runtime with 'unimplemented/strange closure type' error

My program (attached) compiles fine, but at runtime exited with the following error message:

internal error: scavenge: unimplemented/strange closure type 12642 @ 0xd007e0
    (GHC version 6.12.3 for i386_apple_darwin)
    Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
Abort trap

It did this halfway through the set of parallel tasks between lines 150:154.

If it hadn't explicitly said to report this as a GHC bug, I would have assumed it was my mistake. As you can see from the program, I'm not hugely proficient at Haskell and in any case not primarily a programmer. Sorry if that makes it more difficult to trace.