GHC: panic! (compiler bug?)

I just tried to compile the most recent version of Pugs
( www.pugscode.org ), and I got this:

--------------------------------
Compiling Prim             ( src/Prim.hs, src/Prim.o )
ghc.EXE: panic! (the `impossible' happened, GHC version
6.4):
        Maybe.fromJust: Nothing

Please report it as a compiler bug to
glasgow-haskell-bugs@haskell.org,
or http://sourceforge.net/projects/ghc/.
--------------------------------

Well, here I am, reporting this bug. 

The link to the Prim.hs file:
http://svn.openfoundry.org/pugs/src/Prim.hs

I didn't write the code, so can't really tell what's
going on here, the only thing I know is that it did
compile with yesterday's revision (don't know the exact
number; the current svn-revision as I'm writing this is
1572).

I'm running GHC 6.4 on WinXP SP2

Regards,
Martin
mx.2000@gmail.com