[firedrake] boundary conditions on a mixed function space

Dear Firedrakers, I've been trying to implement 0 dirichlet boundaries on a mixed function space that includes a vector space #Set up boundary conditions bcv_x = DirichletBC(W.sub(0).sub(0), Constant(0), method="geometric") bcv_z = DirichletBC(W.sub(0).sub(1), Constant(0), method="geometric") but I get back AttributeError: 'VectorFunctionSpace' object has no attribute 'sub' I thought we were meant to use sub twice to refer first to the function space component and then to the vector component. Also, would this be a correct approach for implementing no normal flow boundaries or is there a more appropriate method to use. Thanks Will