discretizing a zero linear form yields an error

[campospinto]

The following code runs fine:

    domain = Line()
    V = ScalarFunctionSpace('V', domain, kind='H1')
    x = domain.coordinates
    v = element_of(V, name='F')
    f = x
    l  = LinearForm(v, integral(domain, f*v))
    domain_h = discretize(domain, ncells=(2,))
    Vh = discretize(V, domain_h, degree=(2,))
    lh = discretize(l, domain_h, Vh)

but with

    f = x-x

it gives an error

    lh = discretize(l, domain_h, Vh)
  File "/Users/campos/Work/codes/psydac/psydac/feec/multipatch/api.py", line 130, in discretize
    return discretize_single_patch(expr, *args, **kwargs)
  File "/Users/campos/Work/codes/psydac/psydac/api/discretization.py", line 528, in discretize
    return DiscreteLinearForm(a, kernel_expr, *args, **kwargs)
  File "/Users/campos/Work/codes/psydac/psydac/api/fem.py", line 466, in __init__
    BasicDiscrete.__init__(self, expr, kernel_expr, quad_order=quad_order, **kwargs)
  File "/Users/campos/Work/codes/psydac/psydac/api/basic.py", line 257, in __init__
    raise ValueError('> Expecting only one kernel')
ValueError: > Expecting only one kernel

Probably because the 0 expression is overly simplified (TerminalExpr gives ())