derivative of conditional

[tj-sun]

from firedrake import *
# disable_performance_optimisations()

def huber(x, epsilon):
    return conditional(le(x,1),4,x)

m = RectangleMesh(3,3,3,3)
V = VectorFunctionSpace(m, 'CG', 1)

u = Function(V)
v = TestFunction(V)
u.interpolate(Constant((1.0,1.0)))
epsilon = 0.01
E = huber(u[0].dx(0),epsilon)*dx
dE = derivative(E,u,v)
print assemble(dE).vector()[:]

Running the code above in Firedrake throws runtime error:

RuntimeError: Fission error: found unknown node: (t9 ? 0 : (t11[j] * t6) + (t10[j] * t8))

Disabling optimisation fixes the error.