BUG: assign(0, subset=subset) fails

[pbrubeck]

Describe the bug
Failing to assign 0 on a subset.

Steps to Reproduce
Steps to reproduce the behavior:

from firedrake import *

mesh = UnitIntervalMesh(2)

M = FunctionSpace(mesh, "DG", 0)
m = Function(M)
m.dat.data[0] = 1

rmesh = RelabeledMesh(mesh, [m], [100])
V = FunctionSpace(rmesh, "DG", 0)
u = Function(V)

# this works
u.assign(1, subset=rmesh.cell_subset(100))
#this fails
u.assign(0, subset=rmesh.cell_subset(100))

Expected behavior
This should not throw errors.

Error message

Traceback (most recent call last):
  File "/home/pbrubeck/git/dphil_thesis/static_condensation/snippets/bug_subset.py", line 16, in <module>
    u.assign(0, subset=rmesh.cell_subset(100))
  File "petsc4py/PETSc/Log.pyx", line 188, in petsc4py.PETSc.Log.EventDecorator.decorator.wrapped_func
  File "petsc4py/PETSc/Log.pyx", line 189, in petsc4py.PETSc.Log.EventDecorator.decorator.wrapped_func
  File "/home/pbrubeck/firedrake/src/firedrake/firedrake/adjoint_utils/function.py", line 116, in wrapper
    ret = assign(self, other, *args, **kwargs)
  File "/home/pbrubeck/firedrake/src/firedrake/firedrake/function.py", line 458, in assign
    self.dat.zero(subset=subset)
  File "/home/pbrubeck/firedrake/src/firedrake/pyop2/types/dat.py", line 299, in zero
    raise ex.MapValueError("The subset and dataset are incompatible")
pyop2.exceptions.MapValueError: The subset and dataset are incompatible