PETScLUSolver constructor, cpp

The following code snippet -- adapted from the Poisson demo won't compile.

====
  // Create mesh and function space
  UnitSquareMesh mesh(1024, 1024);
  Poisson::FunctionSpace V(mesh);

  // Define boundary condition
  Constant u0(0.0);
  DirichletBoundary boundary;
  DirichletBC bc(V, u0, boundary);

  // Define variational forms
  Poisson::BilinearForm a(V, V);
  Poisson::LinearForm L(V);
  Source f;
  dUdN g;
  L.f = f;
  L.g = g;

  // Compute solution
  Function u(V);

  PETScMatrix A;
  PETScVector b;
  assemble_system(A,b,a,L,bc);

  PETScLUSolver lu(A,"mumps");
  lu.solve(*u.vector(),b);
====

Here's the error:
====
$ make
[100%] Building CXX object CMakeFiles/demo_poisson.dir/main.cpp.o
/home/paulcon/FEniCS/demo/pde/poisson/cpp/main.cpp(96): error: no instance of constructor "dolfin::PETScLUSolver::PETScLUSolver" matches the argument list
            argument types are: (dolfin::PETScMatrix, const char [6])
    PETScLUSolver lu(A,"mumps");
                     ^

compilation aborted for /home/paulcon/FEniCS/demo/pde/poisson/cpp/main.cpp (code 2)
make[2]: *** [CMakeFiles/demo_poisson.dir/main.cpp.o] Error 2
make[1]: *** [CMakeFiles/demo_poisson.dir/all] Error 2
make: *** [all] Error 2
====

What am I missing? The documentation online is unclear, and there aren't any examples of PETScLUSolver in the demos.