Problems with _cell_type assertion

When I try to run demos/twist/twist.py, I receive the following error when using the new Expression/Constant syntax. The code for this branch is available at lp:~cbc-core/cbc.solve/beta.

$ python twist.py
Automatic selection of expression element: <Lagrange vector element of degree ? on a <? of degree 1>: 3 x <CG? on a <? of degree 1>>>
Automatic selection of expression element: <Lagrange vector element of degree ? on a <? of degree 1>: 3 x <CG? on a <? of degree 1>>>
Automatic selection of expression element: <Lagrange vector element of degree ? on a <? of degree 1>: 3 x <CG? on a <? of degree 1>>>
Automatic selection of expression element: <CG? on a <? of degree 1>>
Converting given numerical parameter to DOLFIN Constant
A hyperelastic cube twisted by 60 degrees
Solving nonlinear variational problem
  Starting Newton solve.
[0]PETSC ERROR: ------------------------------------------------------------------------
[0]PETSC ERROR: Caught signal number 10 BUS: Bus Error, possibly illegal memory access
[0]PETSC ERROR: Try option -start_in_debugger or -on_error_attach_debugger
[0]PETSC ERROR: or see http://www.mcs.anl.gov/petsc/petsc-as/documentation/troubleshooting.html#Signal[0]PETSC ERROR: or try http://valgrind.org on linux or man libgmalloc on Apple to find memory corruption errors
[0]PETSC ERROR: configure using --with-debugging=yes, recompile, link, and run
[0]PETSC ERROR: to get more information on the crash.
[0]PETSC ERROR: --------------------- Error Message ------------------------------------
[0]PETSC ERROR: Signal received!
[0]PETSC ERROR: ------------------------------------------------------------------------
[0]PETSC ERROR: Petsc Release Version 3.0.0, Patch 8, Fri Aug 21 14:02:12 CDT 2009
[0]PETSC ERROR: See docs/changes/index.html for recent updates.
[0]PETSC ERROR: See docs/faq.html for hints about trouble shooting.
[0]PETSC ERROR: See docs/index.html for manual pages.
[0]PETSC ERROR: ------------------------------------------------------------------------
[0]PETSC ERROR: Unknown Name on a darwin9.2 named megatr0n.local by hnarayan Mon Dec 7 17:09:45 2009
[0]PETSC ERROR: Libraries linked from /Users/hnarayan/Work/FEniCS/unstable/build/lib
[0]PETSC ERROR: Configure run at Mon Nov 23 17:45:09 2009
[0]PETSC ERROR: Configure options COPTFLAGS=-O3 --with-debugging=0 --with-shared=1 --with-clanguage=cxx --download-umfpack=1 --prefix=/Users/hnarayan/Work/FEniCS/unstable/build
[0]PETSC ERROR: ------------------------------------------------------------------------
[0]PETSC ERROR: User provided function() line 0 in unknown directory unknown file
--------------------------------------------------------------------------
MPI_ABORT was invoked on rank 0 in communicator MPI_COMM_WORLD
with errorcode 59.

NOTE: invoking MPI_ABORT causes Open MPI to kill all MPI processes.
You may or may not see output from other processes, depending on
exactly when Open MPI kills them.
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